Semiconductor device and method of manufacturing semiconductor device

ABSTRACT

To suppress a decrease in on-state current in a semiconductor device including an oxide semiconductor. Provided is a semiconductor device including the following: an oxide semiconductor film which serves as a semiconductor layer; a gate insulating film including an oxide containing silicon, over the oxide semiconductor film; a gate electrode which overlaps with at least the oxide semiconductor film, over the gate insulating film; and a source electrode and a drain electrode which are electrically connected to the oxide semiconductor film. In the semiconductor device, the oxide semiconductor film overlapping with at least the gate electrode includes a region in which a concentration of silicon distributed from the interface with the gate insulating film toward the inside of the oxide semiconductor film is lower than or equal to 1.1 at. %.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device.

In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic device are all semiconductor devices.

2. Description of the Related Art

A technique by which transistors are formed using thin semiconductor films formed over a substrate having an insulating surface has been attracting attention. The transistor is applied to a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A silicon-based semiconductor material is widely known as a material for a thin semiconductor film applicable to a transistor. As another material, an oxide semiconductor material has been attracting attention.

For example, a transistor whose active layer includes an amorphous oxide including indium (In), gallium (Ga), and zinc (Zn) is disclosed (see Patent Document 1).

Transistors including oxide semiconductors have on-state characteristics (e.g., on-state current) superior to those of transistors including amorphous silicon.

In addition, as for such an oxide semiconductor used in a transistor, there is also description as follows: an oxide semiconductor is insensitive to impurities, there is no problem when a considerable amount of metal impurities are contained in a film, and soda-lime glass which contains a large amount of alkali metals such as sodium and is inexpensive can also be used (see Non-Patent Document 1).

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No. 2006-165528

Non-Patent Document

[Non-Patent Document 1] Kamiya, Nomura, and Hosono, “Carrier Transport Properties and Electronic Structures of Amorphous Oxide Semiconductors: The present status”, KOTAI BUTSURI (SOLID STATE PHYSICS), 2009, Vol. 44, pp. 621-633

SUMMARY OF THE INVENTION

However, when a device structure and a process of a transistor including an oxide semiconductor film are designed in accordance with the conventional recognition that an oxide semiconductor is insensitive to impurities, the resistances of source and drain regions are increased or the on-state current is decreased than the designed value, which is a problem.

In view of the problems, an object of one embodiment of the disclosed invention is to improve performance of a transistor including an oxide semiconductor film or a semiconductor device including the transistor. For example, an object is to suppress a decrease in the on-state current of a transistor including an oxide semiconductor film and improve operation characteristics of a semiconductor device including such a transistor.

The present inventors found that the sheet resistance of the oxide semiconductor film is increased by addition of impurities such as silicon to the oxide semiconductor film.

For the purpose of miniaturizing a transistor including an oxide semiconductor film, the oxide semiconductor film is desirably as thin as possible (whereby a short-channel effect can be suppressed, for example). Further, in the case of miniaturizing a transistor, in order to form a source region and a drain region in an oxide semiconductor film in a self-aligned manner, a transistor having a top-gate structure (also referred to as a staggered structure) is generally formed.

In a top-gate structure, a gate insulating film is formed over an oxide semiconductor film. A sputtering method is used as one of methods for forming the gate insulating film. When the gate insulating film is formed by a sputtering method over the oxide semiconductor film, an element ejected from a sputtering target might be taken into the oxide semiconductor film.

In general, a channel region is formed in a portion of an oxide semiconductor film, which is in the vicinity of an interface with a gate insulating film. However, when a constituent element of the gate insulating film is taken into the portion of the oxide semiconductor film, which is in the vicinity of the interface with the gate insulating film, the region unfortunately functions as a high-resistance oxide semiconductor film containing impurities such as silicon. This might cause deterioration of electric characteristics of the transistor, such as a decrease in on-state current.

In view of the foregoing problem, according to one embodiment of the disclosed invention, entry of impurities such as silicon into a portion of an oxide semiconductor film, which is in the vicinity of an interface with a gate insulating film, is suppressed.

One embodiment of the present invention is a semiconductor device including: an oxide semiconductor film; a gate insulating film including an oxide containing silicon, over the oxide semiconductor film; a gate electrode which overlaps with at least the oxide semiconductor film, over the gate insulating film; and a source electrode and a drain electrode which are electrically connected to the oxide semiconductor film. In the semiconductor device, the oxide semiconductor film which overlaps with at least the gate electrode includes a region in which a concentration of silicon distributed from an interface with the gate insulating film toward an inside of the oxide semiconductor film is lower than or equal to 1.1 at. %.

In the above-described structure, a thickness of the region is within 5 nm from the interface with the gate insulating film, and a concentration of silicon contained in a region other than the region is lower than the concentration of silicon contained in the region.

In the above-described structure, the concentration of silicon contained in the region is preferably lower than or equal to 0.83 at. %, more preferably lower than or equal to 0.1 at. %.

In the above-described structure, the gate insulating film may contain carbon, and a concentration of carbon in the region is preferably lower than or equal to 1.0×10²⁰ atoms/cm³.

In the above-described structure, the oxide semiconductor film may have crystallinity, or the oxide semiconductor film may have an amorphous structure.

According to one embodiment of the disclosed invention, a transistor including an oxide semiconductor film or a semiconductor device including the transistor can be improved in performance.

Further, according to one embodiment of the disclosed invention, a decrease in on-state current of a transistor including an oxide semiconductor film can be suppressed, and a semiconductor device including the transistor can have improved operation characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are a plan view and a cross-sectional view, illustrating one embodiment of a semiconductor device.

FIGS. 2A to 2D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 3A to 3D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 4A and 4B are a plan view and a cross-sectional view, illustrating one embodiment of a semiconductor device.

FIGS. 5A to 5E are cross-sectional views illustrating examples of a manufacturing process of a semiconductor device.

FIGS. 6A and 6B are a plan view and a cross-sectional view, illustrating one embodiment of a semiconductor device.

FIGS. 7A to 7C are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 8A and 8B are cross-sectional views illustrating examples of a manufacturing process of a semiconductor device.

FIGS. 9A and 9B are a plan view and a cross-sectional view, illustrating one embodiment of a semiconductor device.

FIGS. 10A to 10C are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 11A to 11C are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 12A and 12B are cross-sectional views illustrating examples of a manufacturing process of a semiconductor device.

FIGS. 13A and 13B are a plan view and a cross-sectional view, illustrating one embodiment of a semiconductor device.

FIGS. 14A to 14C are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 15A to 15C illustrate an example of a structure of a semiconductor device.

FIGS. 16A and 16B illustrate an example of a structure of a semiconductor device.

FIGS. 17A and 17B illustrate an example of a structure of a semiconductor device.

FIGS. 18A and 18B each illustrate an example of a structure of a semiconductor device.

FIG. 19 illustrates an example of a structure of a semiconductor device.

FIG. 20 illustrates an example of a structure of a semiconductor device.

FIG. 21 illustrates an example of a structure of a semiconductor device.

FIGS. 22A to 22F each illustrate an electronic device.

FIG. 23 is a graph showing the result of measurement according to one example of the present invention.

FIGS. 24A to 24C show the result of calculation according to one example of the present invention.

FIGS. 25A and 25B show the result of calculation according to one example of the present invention.

FIGS. 26A and 26B show the result of calculation according to one example of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention disclosed in this specification will be described in detail below with reference to the accompanying drawings. Note that the present invention is not limited to the following description and it will be readily appreciated by those skilled in the art that modes and details can be modified in various ways without departing from the spirit and the scope of the present invention. Therefore, the invention should not be construed as being limited to the description in the following embodiments.

In the following embodiments, the same portions or portions having similar functions are denoted by the same reference numerals in different drawings, and explanation thereof will not be repeated.

Note that the position, the size, the range, or the like of each structure illustrated in the drawings and the like are not accurately represented in some cases for easy understanding. Therefore, the disclosed invention is not necessarily limited to the position, size, range, or the like as disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and the terms do not mean limitation of the number of components.

Note that in this specification and the like, the term such as “over” or “below” does not necessarily mean that a component is placed “directly on” or “directly under” another component. For example, the expression “a gate electrode over a gate insulating layer” can mean the case where there is an additional component between the gate insulating layer and the gate electrode.

In addition, in this specification and the like, the term such as “electrode” or “wiring” does not limit a function of a component. For example, an “electrode” is sometimes used as part of a “wiring”, and vice versa. Furthermore, the term “electrode” or “wiring” can include the case where a plurality of “electrodes” or “wirings” is formed in an integrated manner.

Functions of a “source” and a “drain” are sometimes replaced with each other when a transistor of opposite polarity is used or when the direction of current flowing is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be replaced with each other in this specification and the like.

Note that in this specification and the like, the term “electrically connected” includes the case where components are connected through an object having any electric function. There is no particular limitation on an object having any electric function as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and an element with a variety of functions as well as an electrode and a wiring.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and a method for manufacturing the semiconductor device will be described with reference to FIGS. 1A and 1B, FIGS. 2A to 2D, and FIGS. 3A to 3D.

<Example of Structure of Semiconductor Device>

FIGS. 1A and 1B illustrate an example of a plan view and a cross-sectional view of a transistor having a top-gate structure, as an example of a semiconductor device. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view taken along dashed-dotted line A-B in FIG. 1A. Note that in FIG. 1A, some components (e.g., substrate 100) of a transistor 150 are omitted to avoid complexity.

The transistor 150 shown in FIGS. 1A and 1B includes, over the substrate 100, an insulating film 102, an oxide semiconductor film 106, a gate insulating film 108, a gate electrode 110 which overlaps with at least the oxide semiconductor film, and a source electrode 114 a and a drain electrode 114 b which are electrically connected to the oxide semiconductor film 106.

The oxide semiconductor film 106 can be in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like. Note that the thickness of the oxide semiconductor film 106 is greater than 5 nm and less than or equal to 200 nm, preferably greater than or equal to 10 nm and less than or equal to 30 nm.

Preferably, a CAAC-OS (c-axis aligned crystalline oxide semiconductor) film can be used as the oxide semiconductor film.

The CAAC-OS film is not completely single crystal nor completely amorphous. The CAAC-OS film is an oxide semiconductor film with a crystal-amorphous mixed phase structure where crystal parts and amorphous parts are included in an amorphous phase. Note that in most cases, the crystal part fits inside a cube whose one side is less than 100 nm. From an observation image obtained with a transmission electron microscope (TEM), a boundary between an amorphous part and a crystal part in the CAAC-OS film is not clear. Further, with the TEM, a grain boundary in the CAAC-OS film is not found. Thus, in the CAAC-OS film, a reduction in electron mobility, due to the grain boundary, is suppressed.

In each of the crystal parts included in the CAAC-OS film, a c-axis is aligned in a direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, triangular or hexagonal atomic arrangement which is seen from the direction perpendicular to the a-b plane is formed, and metal atoms are arranged in a layered manner or metal atoms and oxygen atoms are arranged in a layered manner when seen from the direction perpendicular to the c-axis. Note that, among crystal parts, the directions of the a-axis and the b-axis of one crystal part may be different from those of another crystal part. In this specification, a simple term “perpendicular” includes a range from 85° to 95°. In addition, a simple term “parallel” includes a range from −5° to 5°.

In the CAAC-OS film, distribution of crystal parts is not necessarily uniform. For example, in the formation process of the CAAC-OS film, in the case where crystal growth occurs from a surface side of the oxide semiconductor film, the proportion of crystal parts in the vicinity of the surface of the oxide semiconductor film is higher than that in the vicinity of the surface where the oxide semiconductor film is formed in some cases. Further, when an impurity is added to the CAAC-OS film, the crystal part in a region to which the impurity is added becomes amorphous in some cases.

Since the c-axes of the crystal parts included in the CAAC-OS film are aligned in the direction parallel to a normal vector of a surface where the CAAC-OS film is formed or a normal vector of a surface of the CAAC-OS film, the directions of the c-axes may be different from each other depending on the shape of the CAAC-OS film (the cross-sectional shape of the surface where the CAAC-OS film is formed or the cross-sectional shape of the surface of the CAAC-OS film). Note that when the CAAC-OS film is formed, the direction of c-axis of the crystal part is the direction parallel to a normal vector of the surface where the CAAC-OS film is formed or a normal vector of the surface of the CAAC-OS film. The crystal part is formed by film formation or by performing treatment for crystallization such as heat treatment after film formation.

With use of the CAAC-OS film in a transistor, change in electric characteristics of the transistor due to irradiation with visible light or ultraviolet light can be reduced. Thus, the transistor has high reliability.

For example, the CAAC-OS film is formed by a sputtering method with a polycrystalline oxide semiconductor sputtering target. When ions collide with the sputtering target, a crystal region included in the sputtering target may be separated from the target along an a-b plane; in other words, a sputtered particle having a plane parallel to an a-b plane (flat-plate-like sputtered particle or pellet-like sputtered particle) may flake off from the sputtering target. In that case, the flat-plate-like sputtered particle reaches a substrate while maintaining their crystal state, whereby the CAAC-OS film can be formed.

For the deposition of the CAAC-OS film, the following conditions are preferably used.

By reducing the amount of impurities entering the CAAC-OS film during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, or nitrogen) which exist in the deposition chamber may be reduced. Furthermore, the concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, preferably −100° C. or lower is used.

By increasing the substrate heating temperature during the deposition, migration of a sputtered particle is likely to occur after the sputtered particle reaches a substrate surface. Specifically, the substrate heating temperature during the deposition is higher than or equal to 100° C. and lower than or equal to 740° C., preferably higher than or equal to 200° C. and lower than or equal to 500° C. By increasing the substrate heating temperature during the deposition, when the flat-plate-like sputtered particle reaches the substrate, migration occurs on the substrate surface, so that a flat plane of the flat-plate-like sputtered particle is attached to the substrate.

Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol. % or higher, preferably 100 vol. %.

As an example of the sputtering target, an In—Ga—Zn—O compound target is described below.

The In—Ga—Zn—O compound target, which is polycrystalline, is made by mixing InO_(x) powder, GaO_(Y) powder, and ZnO_(Z) powder in a predetermined molar ratio, applying pressure, and performing heat treatment at a temperature higher than or equal to 1000° C. and lower than or equal to 1500° C. Note that X, Y, and Z are each a given positive number. Here, the predetermined molar ratio of InO_(X) powder to GaO_(Y) powder and ZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or 3:1:2. The kinds of powder and the molar ratio for mixing powder may be determined as appropriate depending on the desired sputtering target.

As shown in FIG. 1B, an end of the oxide semiconductor film 106 is preferably tapered at an angle of 20° to 50°. Note that the taper angle is a tilt angle formed by a side surface and a bottom surface of a film having a tapered shape (e.g., oxide semiconductor film 106) in the case where the film is observed from a direction perpendicular to a cross section (a plane perpendicular to the surface of a substrate). If the oxide semiconductor film 106 has a perpendicular end, oxygen is likely to be released from the oxide semiconductor film 106, and accordingly, oxygen vacancies are likely to be generated. In contrast, when the oxide semiconductor film 106 has a tapered end, the generation of oxygen vacancies can be suppressed, and thus, generation of leakage current of the transistor 150 can be reduced.

An oxide semiconductor to be used for the oxide semiconductor film 106 preferably contains at least indium (In) or zinc (Zn). In particular, In and Zn are preferably contained. As a stabilizer for reducing variation in electric characteristics of a transistor including the oxide semiconductor, gallium (Ga) is preferably additionally contained. Tin (Sn) is preferably contained as a stabilizer. It is also preferable that one or more kinds of elements selected from hafnium (Hf), zirconium (Zr), titanium (Ti), scandium (Sc), yttrium (Y), lanthanoid (e.g., cerium (Ce), neodymium (Nd), or gadolinium (Gd)) be contained as a stabilizer.

As the oxide semiconductor, for example, any of the following can be used: an indium oxide; a tin oxide; a zinc oxide; a two-component metal oxide such as an In—Zn-based oxide, a Sn—Zn-based oxide, an Al—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, an In—Mg-based oxide, or an In—Ga-based oxide; a three-component metal oxide such as an In—Ga—Zn-based oxide (also referred to as IGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, a Sn—Ga—Zn-based oxide, an Al—Ga—Zn-based oxide, a Sn—Al—Zn-based oxide, an In—Hf—Zn-based oxide, an In—Zr—Zn-based oxide, an In—Ti—Zn-based oxide, an In—Sc—Zn-based oxide, an In—Y—Zn-based oxide, an In—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide, an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-based oxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, an In—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide, an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-based oxide; and a four-component metal oxide such as an In—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, an In—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, an In—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Zn as its main components and there is no particular limitation on the ratio of In:Ga:Zn. The In—Ga—Zn-based oxide may contain a metal element other than the In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0, m is not an integer) may be used as an oxide semiconductor. Note that M represents one or more metal elements selected from Ga, Fe, Mn, and Co, or the above-described element as a stabilizer. Alternatively, as the oxide semiconductor, a material expressed by a chemical formula, In₂SnO₅(ZnO)_(n) (n>0, n is an integer) may be used.

For example, an In—Ga—Zn-based oxide with an atomic ratio where In:Ga:Zn=1:1:1, In:Ga:Zn=3:1:2, In:Ga:Zn=1:3:2, or In:Ga:Zn=2:1:3, or an oxide whose atomic ratio is in the neighborhood of the above compositions can be used.

For the gate insulating film 108, an oxide insulating film having a sufficient withstand voltage and a sufficient insulating property is preferably used. In the case where the gate insulating film 108 has a single-layer structure, an insulating film including an oxide containing silicon, such as a silicon oxide film, can be used for example.

Alternatively, the gate insulating film 108 may have a stacked-layer structure. In the case where the gate insulating film 108 has a stacked-layer structure, a gallium oxide, an aluminum oxide, a silicon nitride, a silicon oxynitride, an aluminum oxynitride, an yttrium oxide, a lanthanum oxide, a silicon nitride oxide, or the like may be stacked over a silicon oxide. Further, a high-k material such as a hafnium oxide, a hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), a hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0, y>0)), or a hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) may be formed over a silicon oxide. The use of a high-k material enables a reduction in gate leakage current.

When the oxide insulating film is used as the gate insulating film 108, oxygen can be released by heating the oxide insulating film, so that oxygen can be supplied to the oxide semiconductor film 106. Thus, an oxygen vacancy in the oxide semiconductor film 106 can be filled. In particular, the gate insulating film 108 preferably contains a large amount of oxygen which exceeds at least the stoichiometric ratio in (a bulk of) the gate insulating film 108. For example, a film of a silicon oxide represented by the formula SiO_(2+α)(α>0) is preferably used as the gate insulating film 108. When such a silicon oxide film is used as the gate insulating film 108, oxygen can be supplied to the oxide semiconductor film 106, so that the transistor 150 including the oxide semiconductor film 106 can have favorable transistor characteristics.

To release oxygen by heat treatment means that the amount of released oxygen molecules is greater than or equal to 1.0×10¹⁸ molecules/cm³, preferably greater than or equal to 3.0×10¹⁹ molecules/cm³, more preferably 1.0×10²⁰ molecules/cm³ in thermal desorption spectroscopy (TDS).

However, in the case where the silicon oxide film is used as the gate insulating film 108, silicon and the like in the gate insulating film 108 might be taken into the oxide semiconductor film 106 and serve as impurities. Silicon and the like enter the oxide semiconductor film 106 and serve as impurities, which increases the resistance of the oxide semiconductor film 106.

Thus, in the semiconductor device described in this embodiment, entry of impurities such as silicon from the gate insulating film 108 into a portion of the oxide semiconductor film 106, which is in the vicinity of an interface with the gate insulating film 108, is suppressed. Specifically, in the oxide semiconductor film 106, a region in which a concentration of silicon distributed from an interface with the gate insulating film 108 toward the inside of the oxide semiconductor film 106 is lower than or equal to 1.1 at. % is formed. Note that the region is referred to as a region 106 a in this specification and the like. The concentration of silicon contained in the region 106 a is preferably lower than or equal to 0.83 at. %, more preferably lower than or equal to 0.1 at. %. Further, a thick of the region 106 a is preferably within 5 nm from the interface with the gate insulating film 108.

Note that a region in the oxide semiconductor film 106 other than the region 106 a is referred to as a region 106 b. The concentration of silicon contained in the region 106 b is lower than the concentration of silicon contained in the region 106 a.

In the case where impurities such as carbon are contained in the gate insulating film 108, such impurities might also enter the oxide semiconductor film 106 and serve as impurities, as in the case of silicon. Thus, the concentration of carbon contained in the region 106 a is lower than or equal to 1.0×10²⁰ atoms/cm³, preferably lower than or equal to 1.0×10¹⁹ atoms/cm³.

As described above, the amount of impurities such as silicon entering the region 106 a of the oxide semiconductor film 106 is reduced, so that a decrease in on-state current of the transistor 150 including the oxide semiconductor film 106 can be suppressed. Accordingly, the semiconductor device including the transistor 150 can have improved operation characteristics. Further, a transistor including the oxide semiconductor film or a semiconductor device including the transistor can be improved in performance.

Note that the details of the other components are described in description of a method for manufacturing the transistor 150 below, with reference to FIGS. 2A to 2D and FIGS. 3A to 3D.

Note that an insulating film or a planarizing insulating film may further be formed over the transistor 150.

An example of a manufacturing process of the transistor 150 shown in FIGS. 1A and 1B will be described below with reference to FIGS. 2A to 2D and FIGS. 3A to 3D.

<Manufacturing Process of Transistor 150>

First, the substrate 100 having an insulating surface is prepared, and the insulating film 102 is formed over the substrate 100 (see FIG. 2A).

There is no particular limitation on a substrate that can be used as the substrate 100 having an insulating surface as long as it has at least heat resistance high enough to withstand heat treatment performed later. For example, a glass substrate of barium borosilicate glass, aluminoborosilicate glass, or the like, a ceramic substrate, a quartz substrate, a sapphire substrate, or the like can be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate of silicon, silicon carbide, or the like, a compound semiconductor substrate of silicon germanium or the like, an SOI substrate, or the like can be used as long as the substrate has an insulating surface.

A flexible substrate may be used as the substrate 100. In the case of using a flexible substrate, a transistor including the oxide semiconductor film 106 may be directly formed over the flexible substrate, or a transistor including the oxide semiconductor film 106 may be formed over a different manufacturing substrate and then separated to be transferred to the flexible substrate. Note that in order to separate the transistor from the manufacturing substrate to be transferred to the flexible substrate, it is desirable to provide a separation layer between the manufacturing substrate and the transistor including the oxide semiconductor film 106.

The insulating film 102 has a function of preventing diffusion of impurities (e.g., hydrogen and moisture) from the substrate 100 and can be formed to have a single-layer structure or a stacked-layer structure using one or more of a silicon nitride film, a silicon oxide film, a silicon nitride oxide film, and a silicon oxynitride film. When an oxide insulating film is used as the insulating film 102, part of oxygen in the oxide insulating film can be released by heat treatment to be described later. Therefore, oxygen can be supplied to the oxide semiconductor film 106 and thus an oxygen vacancy in the oxide semiconductor film 106 can be filled. In particular, the insulating film 102 preferably contains a large amount of oxygen which exceeds at least the stoichiometric ratio in (a bulk of) the insulating film 102. For example, a film of silicon oxide represented by the formula SiO_(2+α)(α>0) is preferably used as the insulating film 102. When such a silicon oxide film is used as the insulating film 102, oxygen can be supplied to the oxide semiconductor film 106 by heat treatment as described above, so that the transistor 150 including the oxide semiconductor film can have favorable transistor characteristics.

Note that before the formation of the insulating film 102 over the substrate 100 or before the formation of the oxide semiconductor film 106 over the insulating film 102, reverse sputtering in which an argon gas is introduced and plasma is generated is preferably performed to remove powdery substances (also referred to as particle or dust) or organic substances which are attached on the surface of the substrate 100 or the surface of the insulating film 102. The reverse sputtering refers to a method in which a voltage is applied to a substrate side to generate plasma in the vicinity of the substrate to modify a surface. Note that instead of argon, a gas of nitrogen, helium, oxygen, or the like may be used.

Further, in order that hydrogen or water be contained in the oxide semiconductor film 106 as little as possible at the time of forming the oxide semiconductor film 106 over the insulating film 102 in a later step, it is preferable that the substrate provided with the insulating film 102 be preheated in a preheating chamber of a sputtering apparatus as pretreatment for a film formation step of the oxide semiconductor film 106 to eliminate impurities such as hydrogen and moisture which are adsorbed to the substrate 100 and the insulating film 102, and evacuate the preheating chamber. Note that it is effective to combine, as an evacuation unit provided in the preheating chamber, a cryopump having a high capability in removing moisture and a sputter ion pump (also simply referred to as an ion pump) having a high capability in removing hydrogen. At this time, when impurities are removed while an inert gas is introduced, the rate of elimination of moisture or the like, which is difficult to eliminate only by evacuation, can be further increased.

Note that the insulating film 102 is not necessarily provided. For example, when the substrate 100 on which the amount of impurities such as hydrogen and moisture is sufficiently reduced is used, the insulating film 102 may be omitted.

Then, the oxide semiconductor film 106 is formed over the insulating film 102 (see FIG. 2B). The thickness of the oxide semiconductor film 106 is greater than or equal to 1 nm and less than or equal to 50 nm, preferably greater than or equal to 1 nm and less than or equal to 30 nm, more preferably greater than or equal to 1 nm and less than or equal to 10 nm, still more preferably greater than or equal to 3 nm and less than or equal to 7 nm. When the oxide semiconductor film 106 has a thickness in the above range, a short-channel effect of the transistor 150 can be suppressed.

The oxide semiconductor film 106 is in a single crystal state, a polycrystalline (also referred to as polycrystal) state, an amorphous state, or the like as described above. The oxide semiconductor film is preferably a CAAC-OS (c-axis aligned crystalline oxide semiconductor) film.

In this embodiment, the oxide semiconductor film 106 is formed using an In—Ga—Zn-based oxide target by a sputtering method. Further, the oxide semiconductor film 106 can be formed by a sputtering method in a rare gas (typically, argon) atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gas and oxygen.

As a target used for forming an In—Ga—Zn—O film as the oxide semiconductor film 106 by a sputtering method, for example, an oxide target with an atomic ratio where In:Ga:Zn=1:1:1, an oxide target with an atomic ratio where In:Ga:Zn=3:1:2, an oxide target with an atomic ratio where In:Ga:Zn=1:3:2, or an oxide target with an atomic ratio where In:Ga:Zn=2:1:3 can be used. However, a material and composition of a target used for formation of the oxide semiconductor film 106 are not limited to the above.

The relative density of the oxide target is greater than or equal to 90% and less than or equal to 100%, preferably greater than or equal to 95% and less than or equal to 99.9%. By using the oxide target with high relative density, the oxide semiconductor film 106 can be a dense film.

It is preferable that a high-purity gas from which impurities such as hydrogen, water, a hydroxyl group, or a hydride are removed be used as a sputtering gas used for the formation of the oxide semiconductor film 106.

When the oxide semiconductor film 106 contains a large amount of hydrogen, the hydrogen and an oxide semiconductor are bonded to each other, so that part of the hydrogen serves as a donor and causes generation of an electron which is a carrier. As a result, the threshold voltage of the transistor shifts in the negative direction. Accordingly, the hydrogen concentration in the oxide semiconductor film 106 is preferably lower than 5×10¹⁸ atoms/cm³, more preferably lower than or equal to 1×10¹⁸ atoms/cm³, still more preferably lower than or equal to 5×10¹⁷ atoms/cm³, further more preferably lower than or equal to 1×10¹⁶ atoms/cm³. Note that the concentration of hydrogen in the oxide semiconductor film is measured by secondary ion mass spectrometry (SIMS).

When the oxide semiconductor film 106 contains an alkali metal or an alkaline earth metal, the alkali metal or the alkaline earth metal and an oxide semiconductor are bonded to each other, so that carriers are generated in some cases, which causes an increase in the off-state current of the transistor. Accordingly, it is desirable that the concentration of an alkali metal or an alkaline earth metal in the oxide semiconductor film 106 be lower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equal to 2×10¹⁶ atoms/cm³.

The oxide semiconductor film 106 is deposited in the state where the substrate 100 is held in a deposition chamber kept under reduced pressure. At this time, deposition may be performed while the substrate 100 is heated at a substrate temperature higher than or equal to 100° C. and lower than or equal to the strain point of the substrate 100. By heating the substrate 100 during deposition, the concentration of impurities such as hydrogen and moisture in the formed oxide semiconductor film 106 can be reduced (this can be also referred to as dehydration treatment or dehydrogenation treatment). In addition, damage by sputtering can be reduced, which is preferable. Then, a sputtering gas from which hydrogen and moisture are removed is introduced into the deposition chamber where remaining moisture is being removed, and the oxide semiconductor film 106 is deposited with use of the above target, over the substrate 100. In order to remove moisture remaining in the deposition chamber, an entrapment vacuum pump such as a cryopump, an ion pump, or a titanium sublimation pump is preferably used. Further, an evacuation unit may be a turbo pump provided with a cold trap. From the deposition chamber which is evacuated with a cryopump, a hydrogen atom, a compound containing a hydrogen atom such as water (H₂O) (more preferably, also a compound containing a carbon atom), and the like are removed, whereby the concentration of impurities such as hydrogen and moisture in the oxide semiconductor film 106 formed in the deposition chamber can be reduced.

Further, after the formation of the oxide semiconductor film 106, heat treatment may be performed on the oxide semiconductor film 106. The temperature of the heat treatment is higher than or equal to 300° C. and lower than or equal to 700° C., or lower than the strain point of the substrate. The heat treatment is performed, whereby excess hydrogen (including water and a hydroxyl group) can be removed.

The heat treatment can be performed in such a manner that, for example, an object to be processed is introduced into an electric furnace in which a resistance heater or the like is used and heated at 450° C. in a nitrogen atmosphere for an hour. During the heat treatment, the oxide semiconductor film 106 is not exposed to air to prevent entry of water and hydrogen.

The heat treatment apparatus is not limited to the electric furnace and may be an apparatus for heating an object by thermal radiation or thermal conduction from a medium such as a heated gas. For example, a rapid thermal anneal (RTA) apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamp rapid thermal anneal (LRTA) apparatus can be used. An LRTA apparatus is an apparatus for heating an object by radiation of light (an electromagnetic wave) emitted from a lamp such as a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressure sodium lamp, or a high pressure mercury lamp. A GRTA apparatus is an apparatus for performing heat treatment using a high-temperature gas. As the gas, an inert gas which does not react with an object to be processed by heat treatment, such as nitrogen or a rare gas such as argon is used.

For example, as the heat treatment, the GRTA process may be performed as follows. The object is put in a heated inert gas atmosphere, heated for several minutes, and taken out of the inert gas atmosphere. The GRTA process enables high-temperature heat treatment for a short time. Moreover, the GRTA process can be employed even when the temperature exceeds the upper temperature limit of the object. Note that the inert gas may be switched to a gas containing oxygen during the process.

Note that as the inert gas atmosphere, an atmosphere that contains nitrogen or a rare gas (e.g., helium, neon, or argon) as its main component and does not contain water, hydrogen, or the like is preferably used. For example, the purity of nitrogen or a rare gas such as helium, neon, or argon introduced into a heat treatment apparatus is greater than or equal to 6N (99.9999%), preferably greater than or equal to 7N (99.99999%) (that is, the concentration of the impurities is less than or equal to 1 ppm, preferably less than or equal to 0.1 ppm).

The dehydration or dehydrogenation treatment may be accompanied by elimination of oxygen which is a main component material for an oxide semiconductor film to lead to a reduction in oxygen. An oxygen vacancy exists in a portion where oxygen is eliminated in an oxide semiconductor film, and a donor level which leads to a change in the electric characteristics of a transistor is formed owing to the oxygen vacancy. Therefore, in the case where the dehydration or dehydrogenation treatment is performed, oxygen is preferably supplied to the oxide semiconductor film 106. By supply of oxygen to the oxide semiconductor film 106, an oxygen vacancy in the film can be repaired.

The oxygen vacancy in the oxide semiconductor film 106 may be filled in the following manner for example: after the oxide semiconductor film 106 is subjected to the dehydration treatment (the dehydrogenation treatment), a high-purity oxygen gas, a nitrous oxide gas, a high-purity nitrous oxide gas, or ultra dry air (the moisture amount is less than or equal to 20 ppm (−55° C. by conversion into a dew point), preferably less than or equal to 1 ppm, more preferably less than or equal to 10 ppb, in the measurement with the use of a dew point meter of a cavity ring down laser spectroscopy (CRDS) system) may be introduced into the same furnace. It is preferable that the oxygen gas or the nitrous oxide gas do not contain water, hydrogen, and the like. Alternatively, the purity of an oxygen gas or a nitrous oxide gas which is introduced into the heat treatment apparatus is preferably 6N or higher, more preferably 7N or higher (that is, the impurity concentration of the oxygen gas or the nitrous oxide gas is 1 ppm or lower, preferably 0.1 ppm or lower).

Further, instead of the above-described method in which heating is performed in an atmosphere containing oxygen, oxygen may be supplied to the oxide semiconductor film 106 in such a manner that oxygen (including at least any of an oxygen radical, an oxygen atom, and an oxygen ion) is added to the oxide semiconductor film 106. An ion implantation method, an ion doping method, a plasma immersion ion implantation method, plasma treatment, or the like can be used as a method for adding oxygen.

As described above, the oxide semiconductor film 106 deposited is subjected to the dehydration treatment (the dehydrogenation treatment), whereby hydrogen or moisture is removed from the oxide semiconductor so that the oxide semiconductor is purified so as to contain impurities as little as possible. Then, oxygen that is a main component material of the oxide semiconductor, which is reduced through the dehydration treatment (the dehydrogenation treatment), is supplied (also referred to as peroxide treatment), whereby the oxygen vacancy can be filled. In this manner, the oxide semiconductor film 106 can be made to be an i-type (intrinsic) semiconductor or a semiconductor extremely close to an i-type semiconductor. Accordingly, the Fermi level (Ef) of the oxide semiconductor film can be changed to the same level as the intrinsic Fermi level (Ei). Thus, the oxide semiconductor film enables reduction of a variation in the threshold voltage V_(th) of the transistor and a shift of the threshold voltage (ΔV_(th)) due to an oxygen vacancy.

Therefore, it is preferable that the dehydration treatment (the dehydrogenation treatment) be performed before a step of supplying oxygen to the oxide semiconductor film 106.

Incidentally, it is known that oxygen contains isotopes such as ¹⁷O and ¹⁸O and the proportions of ¹⁷O and ¹⁸O in all of the oxygen atoms in nature is about 0.037% and about 0.204%, respectively. That is to say, it is possible to measure the concentrations of these isotopes in the oxide semiconductor film by a method such as SIMS; therefore, the oxygen concentration of the oxide semiconductor film may be able to be estimated more accurately by measuring the concentrations of these isotopes. Thus, the concentrations of these isotopes may be measured to determine whether oxygen is intentionally added to the oxide semiconductor film.

Note that in the above description, the dehydrogenation treatment, the peroxide treatment, and the addition of oxygen are performed before processing of the oxide semiconductor film 106 into an island-like shape; however, one embodiment of the disclosed invention is not construed as being limited thereto. The treatment may be performed after processing of the oxide semiconductor film 106 into an island-like shape.

Next, the oxide semiconductor film 106 is processed into the island-shaped oxide semiconductor film 106 by a photolithography step (see FIG. 2C). A resist mask which is used for the formation of the island-shaped oxide semiconductor film 106 may be formed by an ink-jet method. Formation of the resist mask by an ink-jet method needs no photomask; thus, manufacturing cost can be reduced. Note that the oxide semiconductor film 106 may be etched by either dry etching or wet etching, or by both dry etching and wet etching.

Here, as shown in FIG. 2C, an end of the oxide semiconductor film 106 is preferably tapered at an angle of 20° to 50°. If the oxide semiconductor film 106 has a perpendicular end, oxygen is likely to be released from the oxide semiconductor film 106, and thus, oxygen vacancies are likely to be generated. In contrast, when the oxide semiconductor film 106 has a tapered end, generation of oxygen vacancies can be suppressed, and thus, generation of leakage current of the transistor 150 can be reduced.

Then, an insulating film 107 for forming the gate insulating film 108 is formed over the oxide semiconductor film 106 (see FIG. 2D). Here, the thickness of the insulating film 107 can be, for example, greater than or equal to 1 nm and less than or equal to 50 nm, for example. The insulating film 107 can be formed by, for example, a sputtering method, an MBE method, a CVD method, a pulse laser deposition method, an ALD method, or the like as appropriate.

For the insulating film 107, an oxide insulating film having a sufficient withstand voltage and a sufficient insulating property is preferably used. In the case where the insulating film 107 has a single-layer structure, an insulating film including an oxide containing silicon, such as a silicon oxide film, can be used for example.

Alternatively, the insulating film 107 may have a stacked-layer structure. In the case where the insulating film 107 has a stacked-layer structure, a gallium oxide, an aluminum oxide, a silicon nitride, a silicon oxynitride, an aluminum oxynitride, an yttrium oxide, a lanthanum oxide, a silicon nitride oxide, or the like may be stacked over a silicon oxide. Further, a high-k material such as a hafnium oxide, a hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), a hafnium silicate to which nitrogen is added (HfSiO_(x)N_(y) (x>0, y>0)), or a hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) may be formed over a silicon oxide. The use of a high-k material enables a reduction in gate leakage current.

With the use of the oxide insulating film as the insulating film 107, part of oxygen in the oxide insulating film can be released by heat treatment and supplied to the oxide semiconductor film 106 in a manner similar to that of the insulating film 102. Thus, an oxygen vacancy in the oxide semiconductor film 106 can be filled. For the details of the treatment, the description of the insulating film 102 may be referred to. There is no particular limitation on the timing when the heat treatment is performed on the insulating film 107 as long as it is performed after the formation of the insulating film 107.

In particular, the insulating film 107 preferably contains a large amount of oxygen which exceeds at least the stoichiometric ratio in (a bulk of) the insulating film 107. For example, a film of a silicon oxide represented by the formula SiO_(2+α)(α>0) is preferably used as the insulating film 107. When such a silicon oxide film is used as the insulating film 107, oxygen can be supplied to the oxide semiconductor film 106, so that the transistor 150 including the oxide semiconductor film 106 can have favorable transistor characteristics.

In order that the insulating film 107 be an insulating film containing a large amount of oxygen which exceeds at least the stoichiometric ratio as described above, the insulating film 107 is preferably formed by a sputtering method. In the case where a sputtering method is used, impurities such as hydrogen and moisture in a deposition apparatus are removed as much as possible by the method in which the high-purity gas is used, the method in which the deposition apparatus is baked and the impurities are removed with the use of the evacuation apparatus, the method in which the substrate is preheated, and the like as described above. Thus, the concentration of hydrogen and moisture in the insulating film 107 can be low. Also in terms of the above, it is preferable that the insulating film 107 be formed by a sputtering method.

Note that before the formation of the oxide semiconductor film 106, reverse sputtering in which an argon gas is introduced and plasma is generated is preferably performed to remove powdery substances (also referred to as particle or dust) attached on the surface of the gate insulating film 108. The reverse sputtering refers to a method in which a voltage is applied to a substrate side to generate plasma in the vicinity of the substrate to modify a surface. Note that instead of argon, a gas of nitrogen, helium, oxygen, or the like may be used.

In the case where the insulating film 107 is deposited with the use of the sputtering apparatus as described above, when a constituent element of the insulating film 107, such as silicon, collides with the oxide semiconductor film 106, the element might be mixed into the oxide semiconductor film 106. Such a mixing phenomenon significantly occurs in a portion of the oxide semiconductor film 106, which is in the vicinity of an interface with the insulating film 107, specifically in the above-described region 106 a. In the transistor described in this embodiment or the like, a channel region is formed in a portion of the oxide semiconductor film 106, which is in the vicinity of the gate insulating film 108. When a constituent element of the gate insulating film 108 enters the region and serves as an impurity, on-state characteristics (e.g., on-state current) of the transistor might be deteriorated. Change in characteristics of the oxide semiconductor film 106, which is caused by the entry of impurities such as silicon into the oxide semiconductor film 106, will be described in detail in Example 1.

The mixing of a constituent element of the insulating film 107 into the oxide semiconductor film 106 as described above might occur in a portion of the oxide semiconductor film 106, which is in the vicinity of the interface with the insulating film 107. Thus, part of the insulating film may be deposited under the condition that the impact caused by a constituent element of the insulating film which collides with the oxide semiconductor film is low; after that, in performing the rest of the deposition, the impact of collision may be increased. For example, part of the insulating film 107, which is in the vicinity of the interface with the oxide semiconductor film 106, may be deposited under the condition that a deposition power of an apparatus for depositing the insulating film is low, and then, the rest of deposition of the insulating film 107 may be carried on under the condition that the deposition power is increased. Alternatively, part of the insulating film 107, which is in the vicinity of the interface with the oxide semiconductor film 106, may be deposited under the condition that a deposition pressure of an apparatus for depositing the insulating film is high, and then, the rest of deposition of the insulating film 107 may be carried on under the condition that the deposition pressure is reduced. Further alternatively, part of the insulating film 107, which is in the vicinity of the interface with the oxide semiconductor film 106, may be deposited under the condition that a T-S distance of an apparatus for depositing the insulating film is long, and then, the rest of deposition of the insulating film 107 may be carried on under the condition that the T-S distance is decreased.

For example, when the insulating film 107 is deposited with the use of a sputtering apparatus, it is desirable that the specific value of the deposition power be 10 kW or lower, preferably 1 kW or lower, more preferably 500 W or lower, still more preferably 200 W or lower. Note that when the deposition power is extremely reduced, the deposition rate of the insulating film 107 is decreased. Further, if the deposition power is significantly low, plasma is less likely to be generated in the sputtering apparatus, and the possibility that normal deposition treatment is not performed is increased. Thus, it is desirable that the deposition power be 5% or higher of the maximum power that can be applied in the sputtering apparatus. In considering a reduction in the deposition power, a practitioner may select the optimum power value as appropriate in terms of the performance of the sputtering apparatus, the film thickness of the insulating film 107, and the like so that deposition can be normally performed as long as the manufacturing time of the transistor 150 (takt time) is not significantly affected by the deposition time.

When the insulating film 107 is deposited with the use of a sputtering apparatus, it is desirable that the specific value of the deposition pressure be 0.4 Pa or higher, preferably 1.0 Pa or higher, more preferably 2.0 Pa or higher, still more preferably 5.0 Pa or higher. Note that when the deposition pressure is extremely increased, the quality of a film that is to be deposited tends to deteriorate (e.g., the film is sparse). Thus, it is desirable that the deposition pressure be 100 Pa or lower. In considering an increase in the deposition pressure, a practitioner may select the optimum pressure value as appropriate in terms of characteristics necessary for the insulating film 107 (e.g., field-effect mobility or the like).

When the insulating film 107 is deposited with the use of a sputtering apparatus, it is desirable that the specific value of the T-S distance be 30 mm or more, preferably 50 mm or more, more preferably 100 mm or more, still more preferably 300 mm or more. Note that when the T-S distance is extremely increased, the deposition rate of the insulating film 107 is decreased. Thus, it is preferable that the T-S distance be 500 mm or less. In considering an increase in the T-S distance, a practitioner may select the optimum T-S distance as appropriate as long as the manufacturing time of the transistor 150 (takt time) is not significantly affected by the deposition time.

Note that for the purpose of reducing the impact of a constituent element of the insulating film 107, which collides with the oxide semiconductor film 106, the insulating film 107 may be deposited in a state where one or more of the conditions of deposition power, deposition pressure, and T-S distance are set in the above range.

In the case where as the sputtering apparatus, a magnetron-type sputtering apparatus in which a target and a substrate over which a film is to be deposited are set in substantially parallel to each other (simply the apparatus is also called a magnetron sputtering apparatus) is used, besides a constituent element of the insulating film 107, plasma, a secondary electron, or the like collides with the oxide semiconductor film 106; thus, an element included in the insulating film 107 is more likely to enter the oxide semiconductor film 106. Therefore, as a sputtering apparatus used for depositing the insulating film 107, a facing-target-type sputtering apparatus (also called mirrortron sputtering apparatus, naturatron sputtering apparatus, or the like) may be used. In the apparatus, two targets are set to face each other, a substrate over which a film is to be deposited is set in a portion other than a space sandwiched between the two targets to be in a direction substantially perpendicular to the targets. Then, high-density plasma is generated between the two targets facing each other, and surfaces of the targets (which are used for deposition of the insulating film 107) are sputtered by the plasma, whereby the insulating film 107 is deposited over the substrate. Thus, the substrate where a film is to be deposited is not (or hardly) exposed to plasma or a secondary electron directly.

Further, in the case where the insulating film 107 is deposited by a sputtering method in a rare gas atmosphere, helium may be used instead of argon. The number of atoms of argon is larger than that of helium. Therefore, in the case where argon ions collide with a portion of the oxide semiconductor film 106, which is in the vicinity of the insulating film 107, a bond of the oxide semiconductor film 106 might be cleaved, so that a constituent element of the insulating film 107 might enter the oxide semiconductor film 106 from a portion where the bond is cleaved. The use of helium as a rare gas ion can suppress such cleavage of the bond; therefore, entry of a constituent element of the insulating film 107 into the oxide semiconductor film 106 can be suppressed. Furthermore, after part of the insulating film, which is in the vicinity of an interface with the oxide semiconductor film 106, is deposited in a helium atmosphere, the atmosphere in the deposition chamber may be switched into an argon atmosphere, whereby the deposition rate of the insulating film 107 can be increased.

The insulating film 107 may be deposited by a method by which the impact on the oxide semiconductor film 106 is small, such as an atomic layer deposition (ALD) method, an evaporation method, or a coating method.

As described above, the insulating film 107 is deposited under the condition that the impact of a constituent element of the insulating film 107, which collides with the oxide semiconductor film 106, is reduced, so that in the oxide semiconductor film 106, the region 106 a in which a concentration of silicon distributed from an interface with the insulating film 107 toward the inside of the oxide semiconductor film 106 is lower than or equal to 1.1 at. % and the region 106 b in which a concentration of silicon is lower than that in the region 106 a are formed. Here, the region 106 b indicates a region in the oxide semiconductor film 106 other than the region 106 a. Further, the concentration of silicon contained in the region 106 b is preferably lower than or equal to 0.1 at. %.

Further, when the oxide semiconductor film 106 is deposited in such a manner, entry of impurities such as carbon contained in the insulating film 107 into the oxide semiconductor film 106 can be also reduced. Thus, the concentration of carbon contained in the region 106 a is lower than or equal to 1.0×10²⁰ atoms/cm³, preferably lower than or equal to 1.0×10¹⁹ atoms/cm³.

As described above, the amount of impurities such as silicon entering the region 106 a of the oxide semiconductor film 106 is reduced, so that a decrease in on-state current of the transistor 150 including the oxide semiconductor film 106 can be suppressed. Accordingly, the semiconductor device including the transistor 150 can have improved operation characteristics. Further, a transistor including the oxide semiconductor film or a semiconductor device including the transistor can be improved in performance.

Further, when the insulating film 107 is deposited under the condition that the impact of a constituent element of the insulating film 107, which collides with the oxide semiconductor film 106, is reduced, entry of the constituent element of the oxide semiconductor film 106 into the insulating film 107 can be suppressed. Thus, entry of an element having high conductivity, such as a metal element, of the oxide semiconductor film 106 into the insulating film 107 can be suppressed, and accordingly, a reduction in resistivity of the gate insulating film 108 which is formed using the insulating film 107 can be prevented.

Next, a conductive film 109 for forming the gate electrode 110 (including wirings formed in the same layer as the gate electrode) is formed over the insulating film 107 (see FIG. 3A). The conductive film 109 can be formed using, for example, a metal material such as molybdenum, titanium, tantalum, tungsten, aluminum, copper, neodymium, or scandium, or an alloy material including any of these materials as a main component. Alternatively, the conductive film used for the gate electrode may be formed using a conductive metal oxide. As the conductive metal oxide, an indium oxide (In₂O₃), a tin oxide (SnO₂), a zinc oxide (ZnO), an indium tin oxide (In₂O₃—SnO₂, which is abbreviated to ITO in some cases), an indium zinc oxide (In₂O₃—ZnO), or any of these metal oxide materials in which silicon or a silicon oxide is included can be used. The gate electrode can be formed to have a single-layer structure or a stacked-layer structure using any of the above materials. There is no particular limitation on the method for forming the conductive film, and a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, or a spin coating method can be employed.

As one layer of the gate electrode 110 which is in contact with the gate insulating film, a metal oxide containing nitrogen, specifically, an In—Ga—Zn—O film containing nitrogen, an In—Sn—O film containing nitrogen, an In—Ga—O film containing nitrogen, an In—Zn—O film containing nitrogen, a Sn—O film containing nitrogen, an In—O film containing nitrogen, or a metal nitride (e.g., InN or SnN) film can be used. These films each have a work function of 5 eV or higher, preferably 5.5 eV or higher, which enables the threshold voltage of the transistor to take a positive value when used as the gate electrode layer, so that a switching element of so-called normally-off type can be realized.

Then, by a photolithography step, a resist mask is formed over the conductive film 109, and etching is selectively performed, so that the gate electrode 110 and the gate insulating film 108 are formed; after that, the resist mask is removed (see FIG. 3B). Note that a resist mask which is used for the formation of the gate electrode 110 and the gate insulating film 108 may be formed by an ink-jet method. Formation of the resist mask by an ink-jet method needs no photomask; thus, manufacturing cost can be reduced. Note that the gate electrode 110 and the gate insulating film 108 may be etched by either dry etching or wet etching, or by both dry etching and wet etching.

Next, by an ion doping method or an ion implantation method, an impurity ion 130 having a function of reducing a resistance value of the oxide semiconductor film 106 is added to the oxide semiconductor film 106. At this time, the gate electrode 110 and the gate insulating film 108 function as a mask, whereby a low-resistance region 106 c is formed in a self-aligned manner in the oxide semiconductor film 106 (see FIG. 3C). One or more selected from the following can be used as the impurity ion 130: Group 15 elements (typified by phosphorus (P), arsenic (As), and antimony (Sb)), boron (B), aluminum (Al), nitrogen (N), argon (Ar), helium (He), neon (Ne), indium (In), fluorine (F), chlorine (Cl), titanium (Ti), and zinc (Zn). Note that because an ion implantation method uses a mass separator with which only necessary ion is extracted, only the impurity ion 130 can be selectively added to an object by an ion implantation method. An ion implantation method is thus preferably employed, in which case entry of impurities (e.g., hydrogen) into the oxide semiconductor film 106 is reduced as compared to the case where the ion is added by an ion doping method. Note that the use of an ion doping method is not excluded. Note that change from the first region 106 a and the second region 106 b, which are included in the oxide semiconductor film 106, into the low-resistance region 106 c by the implantation of the impurity ion 130 does not occur. As shown in FIG. 3C, the low-resistance region 106 c includes the region 106 a and the region 106 b.

Next, a conductive film which to be used as a source electrode and a drain electrode (including wirings formed in the same layer as the source electrode and the drain electrode) is formed over the oxide semiconductor film 106. As the conductive film to be used as the source electrode and the drain electrode, for example, a metal film containing an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, or tungsten, or a metal nitride film containing any of the above elements as its component (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) can be used. Alternatively, the conductive film may have a structure in which a film of a high-melting-point metal such as titanium, molybdenum, or tungsten, or a metal nitride film of any of these metals (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) is stacked on either or both of the bottom surface and the top surface of a metal film of aluminum, copper, or the like. Further, the conductive film to be used as the source electrode and the drain electrode may be formed using a conductive metal oxide. As the conductive metal oxide, an indium oxide (In₂O₃), a tin oxide (SnO₂), a zinc oxide (ZnO), an indium tin oxide (In₂O₃—SnO₂, which is abbreviated to ITO), or an indium zinc oxide (In₂O₃—ZnO) can be used. The conductive film to be used as the source electrode and the drain electrode can be formed to have a single-layer structure or a stacked-layer structure using any of the above-described materials. There is no particular limitation on the method for forming the conductive film, and a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, or a spin coating method can be employed.

Then, by a photolithography step, a resist mask is formed over the conductive film, and selective etching is performed, so that the source electrode 114 a and the drain electrode 114 b are formed; after that, the resist mask is removed. Thus, the transistor 150 is manufactured (see FIG. 3D). For the light exposure for forming the resist mask in the photolithography step, ultraviolet, KrF laser, or ArF laser is preferably used. Here, channel length L of the transistor is determined by a distance between a lower end of the source electrode 114 a and a lower end of the drain electrode 114 b that are adjacent to each other over the oxide semiconductor film 106. Therefore, in the case where the channel length L is less than 25 nm, the light exposure at the time of forming the resist mask in the photolithography step is preferably performed using, for example, extreme ultraviolet having an extremely short wavelength of several nanometers to several tens of nanometers. In the light exposure by extreme ultraviolet light, the resolution is high and the focus depth is large. Thus, the channel length L of the transistor formed later can be reduced, whereby the operation speed of a circuit can be increased.

Note that for suppressing a decrease in on-state current of the transistor 150, the following distances are preferably as short as possible: a distance between an end of a portion of the oxide semiconductor film 106 which overlaps with the gate electrode 110, and an end of a portion of the oxide semiconductor film 106 in contact with the source electrode 114 a, which is closer to the gate electrode as compared to the other end thereof (i.e., a distance represented by X in FIG. 3D and FIG. 1A, which is referred to as “Loff width” in this specification); and a distance between the other end of the portion of the oxide semiconductor film 106 which overlaps with the gate electrode 110, and an end of a portion of the oxide semiconductor film 106 in contact with the drain electrode 114 b, which is closer to the gate electrode as compared to the other end thereof (i.e., a distance represented by Y in FIG. 3D and FIG. 1A, which is also referred to as “Loff width” in this specification). The distance X and the distance Y in FIG. 3D may be shortened in such a manner that the source electrode 114 a and the drain electrode 114 b are formed with the use of different photomasks, for example. Thus, at the time of the light exposure, alignment can be performed such that only one of the source electrode 114 a and the drain electrode 114 b is as close to the gate electrode 110 as possible, and therefore, a narrow Loff width can be obtained.

Further, for reducing takt time of the transistor 150 and cost, the number of photomasks and the number of steps in a photolithography step are preferably reduced. In order that the number of masks and the number of steps are reduced, an etching step may be performed with the use of a multi-tone mask which is a light-exposure mask through which light is transmitted to have a plurality of intensities for example. A resist mask formed with the use of a multi-tone mask has a plurality of thicknesses and further can be changed in shape by etching; therefore, the resist mask can be used in a plurality of etching steps for processing into different patterns. Therefore, a resist mask corresponding to at least two kinds or more of different patterns can be formed by one multi-tone mask. Thus, the number of light-exposure masks can be reduced and the number of steps in a corresponding photolithography step can be also reduced, whereby simplification of a process can be realized.

It is preferable that etching conditions be optimized so as not to etch and cut the oxide semiconductor film 106 when the conductive film is etched. However, it is difficult to obtain etching conditions in which only the conductive film is etched and the oxide semiconductor film 106 is not etched at all. In some cases, only part of the oxide semiconductor film 106, e.g., 5% to 50% inclusive, in thickness of the oxide semiconductor film 106 is etched when the conductive film is etched, whereby the oxide semiconductor film 106 having a groove portion (a recessed portion) is formed.

In the case where the conductive film to be used as the source electrode and the drain electrode (including wirings formed in the same layer as the source electrode and the drain electrode) is formed using an oxide semiconductor material, it is necessary for the conductive film to be formed using an oxide semiconductor material which is less etched than the oxide semiconductor film 106 in order that the oxide semiconductor film 106 be etched as little as possible when the conductive film is etched to form the source electrode 114 a and the drain electrode 114 b.

When an oxide semiconductor material is used for the source electrode 114 a and the drain electrode 114 b, an interface between the oxide semiconductor film 106 and each of the source electrode 114 a and the drain electrode 114 b might be unclear depending on a material and a film-formation condition of the oxide semiconductor film 106. In the case where the interface is unclear, a portion which can be referred to as mixture regions or mixture layers of the oxide semiconductor film 106 and each of the source electrode 114 a and the drain electrode 114 b might be formed.

The conductive film to be used as the source electrode 114 a and the drain electrode 114 b can also be formed using a conductive material or a semiconductor material whose resistance is reduced by introduction of an impurity ion.

Through the above-described steps, the transistor 150 is manufactured. In the transistor 150, the amount of impurities (e.g., silicon which is a constituent element of the gate insulating film 108) entering the region 106 a of the oxide semiconductor film 106 due to the formation of the gate insulating film 108 is reduced. Thus, a decrease in on-state current of the transistor 150 can be suppressed. Accordingly, a semiconductor device including the transistor 150 can have improved operation characteristics. Further, a transistor including the oxide semiconductor film or a semiconductor device including the transistor can be improved in performance.

An insulating film may be formed over the transistor 150. The insulating film can be formed using the same material and the same film formation method as the gate insulating film 108; therefore, for the insulating film, the above description of the gate insulating film 108 can be referred to. Note that an aluminum oxide film is highly effective in suppressing entry of impurities such as moisture and hydrogen from outside. Therefore, it is desirable to use an aluminum oxide film or a stacked-layer film including an aluminum oxide film as the insulating film. It is more desirable to use an aluminum oxide film having a film density of 3.2 g/cm³ or higher, preferably 3.6 g/cm³ or higher. Thus, it is possible to suppress entry of impurities such as moisture and hydrogen into the oxide semiconductor film 106.

A planarizing insulating film may be formed over the transistor 150. The planarizing insulating film may be formed in such a manner that a material having an insulating property is applied by a spin coating method, a printing method, a dispensing method, an ink-jet method, or the like, and cure treatment (e.g., heat treatment or light irradiation treatment) is performed depending on the applied material. As the material having an insulating property, for example, an organic resin such as an acrylic resin, a polyimide resin, a polyamide resin, a polyamide-imide resin, or an epoxy resin can be used. Alternatively, it is also possible to use a low-dielectric constant material (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that the planarizing insulating film may be formed by stacking a plurality of insulating films formed of any of these materials. Note that the planarizing insulating film contains comparatively much impurity such as moisture; therefore, the planarizing insulating film is preferably formed over the above-described insulating film (e.g., an aluminum oxide or a stacked-layer film containing an aluminum oxide).

In the manner described above, according to one embodiment of the disclosed invention, the amount of impurities which are to be contained in the portion of the oxide semiconductor film, which is in the vicinity of the interface with the gate insulating film, can be reduced in the semiconductor device including the oxide semiconductor. In addition, according to one embodiment of the disclosed invention, a transistor including an oxide semiconductor film or a semiconductor device including the transistor can be improved in performance.

The structures, methods, and the like which are described in this embodiment can be combined as appropriate with any of the structures, methods, and the like which are described in the other embodiments.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device having a structure different from that described in Embodiment 1 and a method for manufacturing the semiconductor device will be described with reference to FIGS. 4A and 4B and FIGS. 5A to 5E.

<Example of Structure of Semiconductor Device>

FIGS. 4A and 4B illustrate an example of a plan view and a cross-sectional view of a transistor having a top-gate structure, as an example of a semiconductor device. FIG. 4A is a plan view and FIG. 4B is a cross-sectional view taken along line E-F in FIG. 4A. Note that in FIG. 4A, some components (e.g., substrate 100) of a transistor 650 are omitted to avoid complexity.

The transistor 650 of this embodiment is different from the transistor described in Embodiment 1 in that a conductive film 602 is provided in the same plane as an insulating film 102 and is adjacent to the insulating film 102.

In general, in a transistor including an oxide semiconductor film as an active layer, a contact resistance tends to be high in a portion where the oxide semiconductor film is in contact with a conductive film. However, when a transistor has the above-described structure, a source electrode 114 a and a drain electrode 114 b are electrically connected to not only the surface side of an oxide semiconductor film 106 but also the rear surface side thereof. Therefore, a contact resistance between the oxide semiconductor film 106 and the source electrode 114 a and a contact resistance between the oxide semiconductor film 106 and the drain electrode 114 b can be reduced, and variation in contact resistance can be reduced. Thus, it is possible to provide a high-performance transistor in which on-state current is high and variation in threshold voltage is suppressed. Therefore, it can be said that the above-described structure is one of the structures suitable for a transistor including an oxide semiconductor.

<Manufacturing Process of Transistor 650>

An example of a manufacturing process of the transistor 650 shown in FIGS. 4A and 4B will be described with reference to FIGS. 5A to 5E.

First, a conductive film is formed over the substrate 100. In a photolithography step, a resist mask is formed over the conductive film and selective etching is performed, so that the conductive film 602 is formed. Then, the resist mask is removed (see FIG. 5A). For a material of the conductive film 602 and the like, the description of the gate electrode 110 and the source electrode 114 a (or the drain electrode 114 b) in the above embodiment can be referred to.

Then, the insulating film 102 is formed over the substrate 100 and the conductive film 602 (see FIG. 5B). Here, a surface of the insulating film 102 is preferably higher than at least a surface of the conductive film 602. By planarizing treatment to be described later, the surface of the conductive film 602 can be approximately the same in height as the surface of the insulating film 102. Thus, it is possible to avoid a problem such as a break in the oxide semiconductor film 106 due to difference in height between the conductive film 602 and the insulating film 102, which occurs when the oxide semiconductor film is formed in a later step. In addition, the oxide semiconductor film 106 can be extremely thin. Therefore, it can be said that the planarizing treatment is one of effective way to miniaturize a transistor.

The planarizing treatment is performed on the insulating film 102 to form the insulating film 102 having a surface in approximately the same plane as the surface of the conductive film 602 (see FIG. 5C). Note that the planarizing treatment performed on the insulating film 102 is preferably chemical mechanical polishing (CMP) treatment. Here, the CMP treatment is treatment for planarizing a surface of an object to be processed by a combination of chemical and mechanical actions using the surface as a reference. In general, the CMP treatment is treatment in which a polishing cloth is attached to a polishing stage, the polishing stage and the object to be processed are each rotated or swung while a slurry (an abrasive) is supplied between the object to be processed and the polishing cloth, and the surface of the object to be processed is polished by chemical reaction between the slurry and the surface of the object to be processed and by action of mechanical polishing of the object to be processed with the polishing cloth.

The CMP treatment may be performed once or plural times. When the CMP treatment is performed plural times, first polishing is preferably performed with a high polishing rate followed by final polishing with a low polishing rate. By performing polishing steps with different polishing rates in combination, the planarity of the surface of the conductive film 602 and the surface of the insulating film 102 can be further increased.

Further, dry etching treatment or the like may be performed to planarize the insulating film 102. As an etching gas, a chlorine-based gas such as chlorine, boron chloride, silicon chloride, or carbon tetrachloride; a fluorine-based gas such as carbon tetrafluoride, sulfur fluoride, or nitrogen fluoride; oxygen; or the like can be used as appropriate. For example, a dry etching method such as a reactive ion etching (RIE) method, an inductively coupled plasma (ICP) etching method, an electron cyclotron resonance (ECR) etching method, a parallel-plate (capacitively coupled plasma) etching method, a magnetron plasma etching method, a dual-frequency plasma etching method, or a helicon wave plasma etching method can be used. In particular, when the insulating film 102 includes an inorganic insulating material containing a large amount of nitrogen, such as silicon nitride or silicon nitride oxide, it might be difficult to remove the inorganic insulating material containing a large amount of nitrogen only by the CMP treatment; therefore, CMP treatment and dry etching or the like are preferably performed in combination.

Furthermore, plasma treatment or the like may be performed to planarize the insulating film 102. The plasma treatment is performed in such a manner that an inert gas such as an argon gas is introduced into a vacuum chamber and an electric field is applied so that a surface to be processed serves as a cathode. The plasma treatment has a principle similar to that of a plasma dry etching method, and is a simpler method because treatment in a general sputtering deposition chamber is possible by using an inert gas. That is, the plasma treatment is treatment in which the surface to be processed is irradiated with ions of an inert gas and minute unevenness of the surface is reduced by a sputtering effect. Therefore, the plasma treatment is also referred to as “reverse sputtering” in this specification.

Note that the conductive film 602 and the insulating film 102 may be formed in an island-like shape as shown in FIG. 5D. Although the conductive film 602 shown in FIG. 5C has a so-called forward tapered shape in which a distance between the ends becomes shorter toward the upper surface, the conductive film 602 may have a so-called reverse tapered shape in which a distance between the ends becomes longer toward the upper surface as shown in FIG. 5E.

The subsequent steps may be performed referring to FIGS. 2A to 2D and FIGS. 3A to 3D and the description thereof.

Through the above-described steps, the transistor 650 illustrated in FIG. 4B can be manufactured. In the transistor 650, which includes the features described in Embodiment 1, a contact resistance between the oxide semiconductor film 106 and the source electrode 114 a and a contact resistance between the oxide semiconductor film 106 and the drain electrode 114 b can be reduced, and variation in contact resistance can be reduced, as described above. Thus, the transistor with high performance in which on-state current is high and variation in threshold voltage is prevented can be provided. Accordingly, a semiconductor device including the transistor 650 can have improved operation characteristics. Further, a transistor including an oxide semiconductor film or a semiconductor device including the transistor can be improved in performance. Furthermore, as described above, the surface of the conductive film 602 can be approximately the same in height as the surface of the insulating film 102, and the oxide semiconductor film 106 can be extremely thin; therefore, it can be said that the above-described structure is one of the structures suitable for miniaturization of a transistor.

The structures, methods, and the like which are described in this embodiment can be combined as appropriate with any of the structures, methods, and the like which are described in the other embodiments.

Embodiment 3

In this embodiment, one embodiment of a semiconductor device having a structure different from that described in any of the above embodiments and a method for manufacturing the semiconductor device will be described with reference to FIGS. 6A and 6B, FIGS. 7A to 7C, and FIGS. 8A and 8B.

<Example of Structure of Semiconductor Device>

FIGS. 6A and 6B illustrate an example of a plan view and a cross-sectional view of a transistor having a top-gate structure, as an example of a semiconductor device. FIG. 6A is a plan view and FIG. 6B is a cross-sectional view taken along line G-H in FIG. 6A. Note that in FIG. 6A, some components (e.g., substrate 100) of a transistor 850 are omitted to avoid complication.

The transistor 850 shown in FIGS. 6A and 6B includes, over the substrate 100, an insulating film 102, an oxide semiconductor film 106, an insulating film 107, a gate electrode 110 which overlaps with at least the oxide semiconductor film, an interlayer insulating film 800, an interlayer insulating film 802, and a source electrode 114 a and a drain electrode 114 b which are electrically connected to the oxide semiconductor film 106 through openings formed in the insulating film 107, the interlayer insulating film 800, and the interlayer insulating film 802.

The transistor 850 is different from the transistor described in any of the above embodiments in that the insulating film 107 is formed so as to cover the oxide semiconductor film 106 and that the source electrode 114 a and the drain electrode 114 b are electrically connected to the oxide semiconductor film 106 through the openings formed in the insulating film 107, the interlayer insulating film 800, and the interlayer insulating film 802.

The insulating film 107 is formed to cover the oxide semiconductor film 106 in the transistor 850, whereby entry of impurities such as moisture into the oxide semiconductor film 106 can be suppressed. Further, since the insulating film 107 is provided over the oxide semiconductor film 106, it is possible to reduce the occurrence of damage in the oxide semiconductor film 106 (e.g., lattice defects in the oxide semiconductor film 106), which is caused by addition of an impurity ion 130 to the oxide semiconductor film 106.

The transistor 850 has such a structure that the source electrode 114 a and the drain electrode 114 b are electrically connected to the oxide semiconductor film 106 through the openings formed in the insulating film 107, the interlayer insulating film 800, and the interlayer insulating film 802. Thus, after formation of the oxide semiconductor film 106, the oxide semiconductor film 106 is subjected to etching step (e.g., an etching gas and plasma at the time of dry etching or an etching agent at the time of wet etching) only in portions under the openings formed in the insulating film 107, the interlayer insulating film 800, and the interlayer insulating film 802. Therefore, it is possible to suppress contamination of the transistor 850 with a substance generated by the etching treatment (e.g., a metal compound generated by reaction of an etching gas used at the time of the dry etching with a metal element of the oxide semiconductor film 106, which might have electrical conductivity and thus become a leak path between the source electrode 114 a and the drain electrode 114 b). In addition, even when part of the source electrode 114 a and part of the drain electrode 114 b are formed to overlap with the gate electrode 110, the source electrode 114 a and the drain electrode 114 b are not electrically connected to the gate electrode 110 because the interlayer insulating film is provided between the source electrode 114 a and the gate electrode 110 and between the drain electrode 114 b and the gate electrode 110. With such a structure, the source electrode 114 a and the drain electrode 114 b can be formed as close to the gate electrode 110 as possible. Therefore, it can be said that such a structure is one of the structures suitable for miniaturization of a transistor.

<Manufacturing Process of Transistor 850>

An example of a manufacturing process of the transistor 850 shown in FIGS. 6A and 6B will be described with reference to FIGS. 7A to 7C and FIGS. 8A and 8B.

First, the insulating film 102, the oxide semiconductor film 106, and the insulating film 107 are formed over the substrate 100 (see FIG. 7A). Note that the step may be performed referring to FIGS. 2A to 2D and the description thereof.

Next, the gate electrode 110 is formed over the insulating film 107, and the impurity ion 130 is added to the oxide semiconductor film 106 with the gate electrode 110 used as a mask, so that a low-resistance region 106 c is formed in a self-aligned manner in the oxide semiconductor film 106 (see FIG. 7B). Note that the step may be performed referring to FIGS. 3A to 3C and the description thereof.

Next, the interlayer insulating film 800 and the interlayer insulating film 802 are formed over the insulating film 107 and the gate electrode 110 (see FIG. 7C).

The interlayer insulating film 800 can be formed using the same material and the same film formation method as the gate insulating film 108; therefore, the description of the gate insulating film 108 in the above-described embodiment can be referred to. Note that an aluminum oxide film is highly effective in suppressing entry of impurities such as moisture and hydrogen from outside. Therefore, it is desirable to use an aluminum oxide film or a stacked-layer film including an aluminum oxide film as the insulating film. It is more desirable to use an aluminum oxide film having a film density of 3.2 g/cm³ or higher. Thus, entry of impurities such as moisture and hydrogen into the oxide semiconductor film 106 can be suppressed.

The interlayer insulating film 802 may be formed in such a manner that a material having an insulating property is applied by a spin coating method, a printing method, a dispensing method, an ink-jet method, or the like, and cure treatment (e.g., heat treatment or light irradiation treatment) is performed depending on the applied material. As a material having an insulating property, for example, an organic resin such as an acrylic resin, a polyimide resin, a polyamide resin, a polyamide-imide resin, or an epoxy resin can be used. In addition to such resin materials, it is also possible to use a low-dielectric constant material (low-k material), a siloxane-based resin, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or the like. Note that the interlayer insulating film may be formed by stacking a plurality of insulating films formed of any of these materials. Note that the interlayer insulating film contains comparatively much impurity such as moisture; therefore, the interlayer insulating film is preferably formed over the above-described insulating film (e.g., an aluminum oxide or a stacked-layer film containing an aluminum oxide).

Note that a stacked-layer structure including the interlayer insulating film 800 and the interlayer insulating film 802 is formed in this embodiment; however, only one of them may be formed.

Next, the openings are formed in at least part of the insulating film 107, part of the interlayer insulating film 800, and part of the interlayer insulating film 802, which overlap with the oxide semiconductor film. Then, the source electrode 114 a and the drain electrode 114 b which are electrically connected to the oxide semiconductor film 106 through the openings are formed (see FIG. 8A).

Note that the insulating film 107, the interlayer insulating film 800, and the interlayer insulating film 802 may be etched by either dry etching or wet etching, or by both dry etching and wet etching. Note that it is preferable that etching conditions be optimized so that the oxide semiconductor film 106 is not etched and divided when the etching is performed. However, it is difficult to obtain etching conditions in which only the insulating film 107, the interlayer insulating film 800, and the interlayer insulating film 802 are etched and the oxide semiconductor film 106 is not etched at all. In some cases, only part of the oxide semiconductor film 106, e.g., 5% to 50% inclusive, in thickness of the oxide semiconductor film 106 is etched when the conductive film is etched, whereby the oxide semiconductor film 106 having a groove portion (a recessed portion) is formed.

Note that a step of forming the source electrode 114 a and the drain electrode 114 b may be performed referring to FIG. 3D and the description thereof. Further, after the step, planarizing treatment may be performed on the source electrode 114 a, the drain electrode 114 b, and the interlayer insulating film 802. By the planarizing treatment, in the case where another transistor is stacked over the transistor 850, the transistor can be easily manufactured. This is because the planarity of a surface over which the transistor is to be formed (i.e., surfaces of the source electrode 114 a, the drain electrode 114 b, and the interlayer insulating film 802) is increased by the planarizing treatment. Note that for the planarizing treatment, a method of the planarizing treatment described in any of the above embodiments can be referred to.

Through the above-described steps, the transistor 850 shown in FIG. 6B can be manufactured. In the transistor 850, which includes the features described in Embodiment 1, the occurrence of damage in the oxide semiconductor film 106 (e.g., lattice defects in the oxide semiconductor film 106) which is caused by addition of an ion can be reduced as described above. Further, as described above, a portion of the oxide semiconductor film 106, which is subjected to etching treatment, can be limited. Therefore, contamination of the transistor by the etching treatment can be suppressed. Accordingly, a semiconductor device including the transistor 850 can have improved operation characteristics. Further, a transistor including an oxide semiconductor film or a semiconductor device including the transistor can be improved in performance. Furthermore, as described above, the source electrode 114 a and the drain electrode 114 b are not electrically connected to the gate electrode 110 even when parts thereof are formed to overlap with the gate electrode 110. Therefore, the source electrode 114 a and the drain electrode 114 b can be formed as close to the gate electrode 110 as possible, which is one of the structures suitable for miniaturization of a transistor.

Further, as shown in FIG. 8B, the conductive film 602 may be included in the insulating film 102. The transistor 850 having the structure shown in FIG. 8B can achieve the following: even when portions of the oxide semiconductor film 106, which are located under the openings, are over-etched and thus eliminated at the time of forming the openings in part of the insulating film 107, part of the interlayer insulating film 800, and part of the interlayer insulating film 802, the source electrode 114 a and the drain electrode 114 b are electrically connected not only to the oxide semiconductor film 106 through the conductive film 602 but also to a sidewall portion of the oxide semiconductor film 106. Thus, a favorable contact resistance can be kept even when such over-etching is performed. Therefore, it can be said that the structure shown in FIG. 8B is particularly suitable for the case where the oxide semiconductor film 106 is thin (i.e., miniaturization of a transistor).

Embodiment 4

In this embodiment, one embodiment of a semiconductor device having a structure different from that described in any of the above embodiments and a method for manufacturing the semiconductor device will be described with reference to FIGS. 9A and 9B, FIGS. 10A to 10C, FIGS. 11A to 11C, and FIGS. 12A and 12B.

<Example of Structure of Semiconductor Device>

FIGS. 9A and 9B illustrate an example of a plan view and a cross-sectional view of a transistor having a top-gate structure, as an example of a semiconductor device. FIG. 9A is a plan view and FIG. 9B is a cross-sectional view taken along line I-J in FIG. 9A. Note that in FIG. 9A, some components (e.g., substrate 100) of a transistor 1150 are omitted to avoid complication.

The transistor 1150 shown in FIGS. 9A and 9B includes, over the substrate 100, an insulating film 102, an oxide semiconductor film 106, a gate insulating film 108, a gate electrode 110 which overlaps with at least the oxide semiconductor film, an insulating film 1101, a sidewall insulating film 1102, and a source electrode 114 a and a drain electrode 114 b which are electrically connected to the oxide semiconductor film 106.

The transistor 1150 is different from the transistor described in any of the above embodiments in the following points: the insulating film 1101 is provided over the gate electrode 110; the sidewall insulating film 1102 is provided on a side surface of the gate electrode 110; and the source electrode 114 a and the drain electrode 114 b are provided to be in contact with the sidewall insulating film 1102.

The transistor 1150 is manufactured in the following manner: a conductive film to be used as the source electrode 114 a and the drain electrode 114 b is formed over the oxide semiconductor film 106, the insulating film 1101, and the sidewall insulating film 1102, and then, part of the conductive film is removed by planarizing treatment (also referred to as polishing treatment) performed on the conductive film, so that the source electrode 114 a and the drain electrode 114 b are formed. This manner is also described later in the description of a method for manufacturing the transistor 1150. Thus, it is not necessary to use a photolithography step for the formation of the source electrode 114 a and the drain electrode 114 b, and a Loff width can be extremely narrow without regard to the accuracy of a light-exposure machine or the misalignment of a photomask. Thus, a decrease in on-state current of the transistor 1150 can be suppressed. Further, it can be said that the above-described structure is one of the structures suitable for miniaturization of a transistor.

<Manufacturing Process of Transistor 1150>

An example of a manufacturing process of the transistor 1150 shown in FIGS. 9A and 9B will be described with reference to FIGS. 10A to 10C and FIGS. 11A to 11C.

First, the insulating film 102, the oxide semiconductor film 106, and an insulating film 107 are formed over the substrate 100 (see FIG. 10A). Note that the formation steps may be performed referring to FIGS. 2A to 2D and the description thereof.

Next, a conductive film 109 for forming the gate electrode 110 (including wirings formed in the same layer as the gate electrode) and an insulating film 1100 for forming the insulating film 1101 are formed (see FIG. 10B). The insulating film 1100 can be formed using the same material and the same film formation method as the gate insulating film 108; therefore, the description of the gate insulating film 108 in any of the above embodiments can be referred to for the insulating film 1100.

Next, in a photolithography step, the conductive film 109 and the insulating film 1100 are processed into island-like shapes, so that the gate electrode 110 and the insulating film 1101 are formed (see FIG. 10C). The resist mask which is used for the formation of the gate electrode 110 and the insulating film 1101 may be formed by an ink-jet method. Formation of the resist mask by an ink-jet method needs no photomask; thus, manufacturing cost can be reduced. Note that the conductive film 109 and the insulating film 1100 may be etched by either dry etching or wet etching, or by both dry etching and wet etching.

In this embodiment, description is made in the following order: the conductive film 109 and the insulating film 1100 are formed and processed together to form the gate electrode 110 and the insulating film 1101; then, the sidewall insulating film 1102 is formed. Therefore, the insulating film 1101 and the sidewall insulating film 1102 are described as different components as shown in FIG. 9B. However, the insulating film 1101 and the sidewall insulating film 1102 may be one film. In the case where the insulating film 1101 and the sidewall insulating film 1102 are formed using one film, after the gate electrode 110 is formed, an insulating film which functions as the insulating film 1101 and the sidewall insulating film 1102 may be formed to cover the gate electrode 110.

Next, by an ion doping method or an ion implantation method, an impurity ion 130 having a function of reducing a resistance value of the oxide semiconductor film 106 is added to the oxide semiconductor film 106. At this time, the gate electrode 110 and the insulating film 1101 function as a mask, whereby a low-resistance region 106 c is formed in a self-aligned manner in the oxide semiconductor film 106 (see FIG. 11A).

Next, an insulating film is formed using the same material and the same method as the insulating film 102, and the insulating film is etched, so that the sidewall insulating film 1102 is formed. The sidewall insulating film 1102 can be formed in a self-aligned manner by performing a highly anisotropic etching step on the insulating film. For example, a dry etching method is preferably employed. As an etching gas used for the dry etching method, for example, a gas including fluorine such as trifluoromethane, octafluorocyclobutane, or tetrafluoromethane can be used. A rare gas or hydrogen may be added to the etching gas. As the dry etching method, a reactive ion etching (RIE) method in which high-frequency voltage is applied to a substrate, is preferably used.

After the formation of the sidewall insulating film 1102, the insulating film 107 is processed with the gate electrode 110, the insulating film 1101, and the sidewall insulating film 1102 used as a mask, so that the gate insulating film 108 can be formed (see FIG. 11B). Note that the gate insulating film 108 may be formed in the same step as the formation of the sidewall insulating film 1102.

Note that although the impurity ion 130 is added to the oxide semiconductor film 106 with the gate electrode 110 and the insulating film 1101 used as a mask just after the formation of the gate electrode 110 and the insulating film 1101 in this embodiment, the impurity ion 130 may be added to the oxide semiconductor film 106 after the formation of the sidewall insulating film 1102, with the gate electrode 110, the insulating film 1101, and the sidewall insulating film 1102 used as a mask.

Then, a conductive film 1104 for forming the source electrode 114 a and the drain electrode 114 b (including wirings formed in the same layer as the source electrode and the drain electrode) is formed over the oxide semiconductor film 106, the insulating film 1101, and the sidewall insulating film 1102, and an interlayer insulating film 802 is formed over the conductive film 1104 (see FIG. 11C). As the conductive film 1104, for example, a metal film containing an element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, or tungsten, or a metal nitride film containing any of the above elements as its component (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) can be used. Alternatively, the conductive film 1104 may have a structure in which a film of a high-melting-point metal such as titanium, molybdenum, or tungsten, or a metal nitride film of any of these metals (a titanium nitride film, a molybdenum nitride film, or a tungsten nitride film) is stacked on either or both of the bottom surface and the top surface of a metal film of aluminum, copper, or the like. Further, the conductive film used for the source electrode and the drain electrode may be formed using a conductive metal oxide. As the conductive metal oxide, an indium oxide (In₂O₃), a tin oxide (SnO₂), a zinc oxide (ZnO), an indium tin oxide (In₂O₃—SnO₂, which is abbreviated to ITO), or an indium zinc oxide (In₂O₃—ZnO) can be used. The conductive film used for the source electrode and the drain electrode can be formed to have a single-layer structure or a stacked-layer structure using any of the above-described materials. There is no particular limitation on the method for forming the conductive film, and a variety of film formation methods such as an evaporation method, a CVD method, a sputtering method, or a spin coating method can be employed. Note that for the interlayer insulating film 802, the materials and the film formation methods of the interlayer insulating film 802 which are described in Embodiment 3 can be referred to.

Then, planarizing treatment is performed on a top surface of the conductive film 1104 to remove at least part of the conductive film 1104 and at least part of the interlayer insulating film 802, which are located over the insulating film 1101 and the sidewall insulating film 1102. Thus, the conductive film 1104 is divided at least over the insulating film 1100 or the sidewall insulating film 1102, so that the gate electrode 110 is interposed between the source electrode 114 a and the drain electrode 114 b (see FIG. 12A). Note that for the planarizing treatment here, the description of the planarizing treatment performed on the insulating film 102 in Embodiment 1 can be referred to.

Note that the planarizing treatment may be performed not only on the conductive film 1104 and the interlayer insulating film 802 but also on the insulating film 1101 and the sidewall insulating film 1102.

Note that in FIG. 12A, surfaces of the source electrode 114 a and the drain electrode 114 b are located in the same plane as surfaces of the insulating film 1101 and the interlayer insulating film 802. However, in the case where the source electrode 114 a, the drain electrode 114 b, and the insulating film 1101 are polished with a CMP apparatus, when the source electrode 114 a and the drain electrode 114 b are polished at a polishing speed different from that of the insulating film 1101 and the interlayer insulating film 802, the surfaces of the source electrode 114 a and the drain electrode 114 b might be different in height from the surfaces of the insulating film 1101 and the interlayer insulating film 802, whereby a step might be formed. For example, the surfaces of the source electrode 114 a and the drain electrode 114 b might be lower than the surface of the insulating film 1101 (i.e., the surfaces of the source electrode 114 a and the drain electrode 114 b might be recessed).

Through the above-described steps, the transistor 1150 shown in FIG. 12A can be manufactured. In the transistor 1150, which includes the features described in Embodiment 1, a decrease in on-state current can be suppressed as described above. Accordingly, a semiconductor device including the transistor 1150 can have improved operation characteristics. Further, a transistor including an oxide semiconductor film or a semiconductor device including the transistor can be improved in performance. Furthermore, it is not necessary to use a photolithography step for the formation of the source electrode 114 a and the drain electrode 114 b, and a Loff width can be extremely narrow without regard to the accuracy of a light-exposure machine or the misalignment of a photomask. Therefore, it can be said that the above-described structure is one of the structures suitable for miniaturization of a transistor.

An insulating film may be formed over the transistor 1150. The insulating film can be formed using the same material and the same film formation method as the gate insulating film 108; thus, the description of the gate insulating film 108 can be referred to for the insulating film. Note that an aluminum oxide film is highly effective in suppressing entry of moisture from outside. Therefore, it is desirable to use an aluminum oxide film or a stacked-layer film including an aluminum oxide film as the insulating film. It is more desirable to use an aluminum oxide film having a film density of 3.2 g/cm³ or higher, preferably 3.6 g/cm³ or higher. Note that the insulating film may be formed before the formation of the transistor 1150. For example, the conductive film 1104, the insulating film, and the interlayer insulating film 802 may be formed in the order presented after the formation of the sidewall insulating film 1102, and then, the planarizing treatment such as CMP may be formed. The structure shown in FIG. 9B is preferable because, even when impurities such as moisture or hydrogen enter the interlayer insulating film 802, the impurities can be prevented from reaching the oxide semiconductor film 106.

The transistor 1150 may have a structure in which the conductive film 602 is included in the insulating film 102 as shown in FIG. 12B. When the transistor 1150 has the structure shown in FIG. 12B, the source electrode 114 a and the drain electrode 114 b are electrically connected to not only the surface side of the oxide semiconductor film 106 but also the rear surface side thereof. Therefore, a contact resistance between the oxide semiconductor film 106 and the source electrode 114 a and a contact resistance between the oxide semiconductor film 106 and the drain electrode 114 b can be reduced, and variation in contact resistance can be reduced. Thus, it is possible to provide a high-performance transistor in which on-state current is high and variation in threshold voltage is prevented. Therefore, it can be said that the above-described structure is one of the structures suitable for a transistor including an oxide semiconductor.

Embodiment 5

In this embodiment, one embodiment of a semiconductor device having a structure different from that described in any of the above embodiments and a method for manufacturing the semiconductor device will be described with reference to FIGS. 13A and 13B and FIGS. 14A to 14C.

<Example of Structure of Semiconductor Device>

FIGS. 13A and 13B illustrate an example of a plan view and a cross-sectional view of a transistor having a top-gate structure, as an example of a semiconductor device. FIG. 13A is a plan view and FIG. 13B is a cross-sectional view taken along line K-L in FIG. 13A. Note that in FIG. 13A, some components (e.g., substrate 100) of a transistor 1350 are omitted to avoid complication.

The transistor 1350 shown in FIGS. 13A and 13B includes, over the substrate 100, an insulating film 102, an oxide semiconductor film 106, a source electrode 114 a and a drain electrode 114 b which are electrically connected to the oxide semiconductor film 106, a gate insulating film 108, and a gate electrode 110 which overlaps with at least the oxide semiconductor film.

The transistor 1350 is different from the transistor described in any of the above embodiments in that the gate insulating film 108 is formed over an entire surface of the oxide semiconductor film 106.

With the structure in which the gate insulating film is formed only over part of the oxide semiconductor film 106 as in the above embodiment, oxygen (excess oxygen in the gate insulating film 108) is released from an end of the gate insulating film 108 even if the gate insulating film 108 is a film which releases oxygen by heat treatment. Thus, an effect of reducing an oxygen vacancy in the oxide semiconductor film 106 might be reduced.

However, with the structure in which the gate insulating film 108 is formed over the entire surface of the oxide semiconductor film 106 as described in this embodiment, oxygen released by heat treatment can be prevented from being released from the end of the gate insulating film 108. Accordingly, the above-described problem can be solved.

<Manufacturing Process of Transistor 1350>

An example of a manufacturing process of the transistor 1350 shown in FIGS. 13A and 13B will be described with reference to FIGS. 14A to 14C.

First, the insulating film 102 and the oxide semiconductor film 106 are formed over the substrate 100 (see FIG. 14A). Note that the step may be performed referring to FIGS. 2A to 2D and the description of the above embodiment which corresponds to the description of the drawings.

Next, the source electrode 114 a and the drain electrode 114 b which are in contact with the oxide semiconductor film 106 are formed, and the gate insulating film 108 is formed over the oxide semiconductor film 106, the source electrode 114 a, and the drain electrode 114 b (see FIG. 14B). Note that the formation of the source electrode 114 a and the drain electrode 114 b may be performed referring to FIG. 3D and the description thereof, and the formation of the gate insulating film 108 may be performed referring to FIG. 2D and the description thereof.

Note that in this embodiment, a portion where a first region 106 a is formed is different from that in any of the above embodiments, because in this embodiment, the gate insulating film 108 is formed after the formation of the source electrode 114 a and the drain electrode 114 b over the oxide semiconductor film 106. In this embodiment, the first region 106 a is formed only in a region of the oxide semiconductor film 106, which is in direct contact with the gate insulating film 108.

Then, the gate electrode 110 is formed in a region over the gate insulating film 108, which overlaps with the oxide semiconductor film 106 (see FIG. 14C). Note that the step may be performed referring to FIG. 3B and the description thereof.

Through the above-described steps, the transistor 1350 illustrated in FIG. 14C can be manufactured. In the transistor 1350, which includes the features described in Embodiment 1, an effect of reducing an oxygen vacancy can be enhanced because oxygen released from the gate insulating film 108 can be efficiently added to the oxide semiconductor film 106 when the gate insulating film 108 is formed using a film which releases oxygen by heat treatment, as described above.

Embodiment 6

In this embodiment, an example of a semiconductor device which includes the transistor described in any of Embodiments 1 to 5, which can hold stored data even when not powered, and which has an unlimited number of write cycles is described with reference to drawings.

FIGS. 15A to 15C illustrate an example of a structure of a semiconductor device. FIG. 15A is a cross-sectional view of the semiconductor device, FIG. 15B is a plan view of the semiconductor device, and FIG. 15C is a circuit diagram of the semiconductor device. Here, FIG. 15A corresponds to a cross section along line K-L and line M-N in FIG. 15B.

The semiconductor device illustrated in FIGS. 15A and 15B includes a transistor 1760 including a first semiconductor material in a lower portion, and a transistor 1762 including a second semiconductor material in an upper portion. Any of the structures of the transistors described in the above embodiments can be employed for the transistor 1762. Here, description is made on the case where the transistor 1150 of Embodiment 4 is used.

Here, the first semiconductor material and the second semiconductor material are preferably materials having different band gaps. For example, the first semiconductor material may be a semiconductor material other than an oxide semiconductor (e.g., silicon) and the second semiconductor material may be an oxide semiconductor. A transistor including a material other than an oxide semiconductor can operate at high speed easily. On the other hand, charge can be held in a transistor including an oxide semiconductor for a long time owing to its characteristics.

Although all the transistors are n-channel transistors here, it is needless to say that p-channel transistors can be used. The specific constituent of the semiconductor device is not necessarily limited to those described here such as the material used for the semiconductor device and the structure of the semiconductor device (e.g., the use of the transistor described in Embodiment 4, which is formed using an oxide semiconductor, as the transistor 1762 for holding information).

The transistor 1760 in FIG. 15A includes a channel formation region 1716 provided in a substrate 1700 containing a semiconductor material (e.g., silicon), impurity regions 1720 provided so that the channel formation region 1716 is sandwiched therebetween, intermetallic compound regions 1724 in contact with the impurity regions 1720, a gate insulating film 1708 provided over the channel formation region 1716, and a gate electrode 1710 provided over the gate insulating film 1708. Note that a transistor whose source electrode and drain electrode are not illustrated in a drawing may be referred to as a transistor for the sake of convenience. Further, in such a case, in description of a connection of a transistor, a source region and a source electrode are collectively referred to as a “source electrode”, and a drain region and a drain electrode are collectively referred to as a “drain electrode”. That is, in this specification, the term “source electrode” may include a source region.

Further, an element isolation insulating layer 1706 is formed over the substrate 1700 to surround the transistor 1760, and an insulating layer 1728 and an insulating layer 1730 are formed to cover the transistor 1760. Note that, in the transistor 1760, the sidewall insulating layers may be formed on side surfaces of the gate electrode 1710 and the impurity regions 1720 may include a region having a different impurity concentration.

The transistor 1760 formed using a single crystal semiconductor substrate can operate at high speed. Thus, when the transistor is used as a reading transistor, data can be read at a high speed. Two insulating films are formed to cover the transistor 1760. As treatment prior to formation of the transistor 1762 and a capacitor 1764, CMP treatment is performed on the two insulating films, whereby an insulating layer 1728 and an insulating layer 1730 which are planarized are formed and, at the same time, a top surface of the gate electrode 1710 is exposed.

As each of the insulating layer 1728 and the insulating layer 1730, typically, it is possible to use an inorganic insulating film such as a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, an aluminum oxynitride film, a silicon nitride film, an aluminum nitride film, a silicon nitride oxide film, or an aluminum nitride oxide film. The insulating layer 1728 and the insulating layer 1730 can be formed by a plasma CVD method, a sputtering method, or the like.

Alternatively, an organic material such as a polyimide, an acrylic resin, or a benzocyclobutene-based resin can be used. Other than such organic materials, it is also possible to use a low-dielectric constant material (a low-k material) or the like. In the case of using an organic material, a wet process such as a spin coating method or a printing method may be used to form the insulating layer 1728 and the insulating layer 1730.

Note that in this embodiment, a silicon nitride film is used as the insulating layer 1728, and a silicon oxide film is used as the insulating layer 1730.

Planarization treatment is preferably performed on the surface of the insulating layer 1730 in the formation region of the oxide semiconductor film 1744. In this embodiment, the oxide semiconductor film 1744 is formed over the insulating layer 1730 sufficiently planarized by polishing treatment such as CMP treatment (the average surface roughness of the surface of the insulating layer 1730 is preferably less than or equal to 0.15 nm).

The transistor 1762 illustrated in FIG. 15A includes an oxide semiconductor in the channel formation region. Here, an oxide semiconductor film 1744 included in the transistor 1762 is preferably highly purified by removing impurities such as moisture and hydrogen as much as possible, as described in the above embodiment. Further, the oxide semiconductor film in which oxygen vacancies are sufficiently filled is preferable. By using such an oxide semiconductor, the transistor 1762 which has extremely favorable off-state current characteristics can be obtained.

Since the off-state current of the transistor 1762 is extremely small, stored data can be held for a long time owing to such a transistor. In other words, power consumption can be sufficiently reduced because a semiconductor device in which refresh operation is unnecessary or the frequency of refresh operation is extremely low can be provided.

In the process for manufacturing the transistor 1762, a conductive film over a gate electrode 1748, an insulating film 1737, a sidewall insulating film 1736 a, and a sidewall insulating film 1736 b is removed by chemical mechanical polishing treatment to form an electrode film 1742 a and an electrode film 1742 b which function as a source electrode and a drain electrode.

Accordingly, in the transistor 1762, a Loff width can be narrow; thus, the on-state characteristics of the transistor 1762 can be improved.

Further, precise processing can be performed accurately because an etching step using a resist mask is not performed in a step for removing the conductive film over the gate electrode 1748, which is one step of the formation process of the electrode film 1742 a and the electrode film 1742 b. Consequently, in a process for manufacturing the semiconductor device, a transistor having a miniaturized structure with less variation in shape or characteristics can be manufactured with high yield.

An interlayer insulating film 1735 and an insulating film 1750 each of which has a single-layer structure or a stacked-layer structure are provided over the transistor 1762. In this embodiment, an aluminum oxide film is used as the insulating film 1750. When the aluminum oxide film has high density (the film density is higher than or equal to 3.2 g/cm³, preferably higher than or equal to 3.6 g/cm³), the transistor 1762 can have stable electric characteristics.

In addition, a conductive layer 1753 is provided in a region overlapping with the electrode film 1742 a of the transistor 1762 with the interlayer insulating film 1735 and the insulating film 1750 interposed therebetween, and the electrode film 1742 a, the interlayer insulating film 1735, the insulating film 1750, and the conductive layer 1753 form a capacitor 1764. That is, the electrode film 1742 a of the transistor 1762 functions as one electrode of the capacitor 1764, and the conductive layer 1753 functions as the other electrode of the capacitor 1764. Note that the capacitor 1764 may be omitted if a capacitor is not needed. Alternatively, the capacitor 1764 may be separately provided above the transistor 1762.

An insulating film 1752 is provided over the transistor 1762 and the capacitor 1764. In addition, a wiring 1756 for connecting the transistor 1762 to another transistor is provided over the insulating film 1752. Although not illustrated in FIG. 15A, the wiring 1756 is electrically connected to the electrode film 1742 b through an electrode formed in an opening provided in the interlayer insulating film 1735, the insulating film 1750, the insulating film 1752, and the like. Here, the electrode is preferably provided to partly overlap with at least part of the oxide semiconductor film 1744 of the transistor 1762.

In FIGS. 15A and 15B, the transistor 1760 is provided to overlap with at least part of the transistor 1762. The source region or the drain region of the transistor 1760 is preferably provided to overlap with part of the oxide semiconductor film 1744. Further, the transistor 1762 and the capacitor 1764 are provided to overlap with at least part of the transistor 1760. For example, the conductive layer 1753 of the capacitor 1764 is provided to overlap with at least part of the gate electrode 1710 of the transistor 1760. With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, higher integration can be achieved.

Note that the electrical connection between the electrode film 1742 b and the wiring 1756 may be established by contacting the electrode film 1742 b with the wiring 1756 directly or may be established through an electrode provided in an insulating film which is between the electrode film 1742 b and the wiring 1756. Alternatively, the electrical connection may be established through a plurality of electrodes.

Next, an example of a circuit configuration corresponding to FIGS. 15A and 15B is illustrated in FIG. 15C.

In FIG. 15C, a first line (1st Line) is electrically connected to a source electrode of the transistor 1760. A second line (2nd Line) is electrically connected to a drain electrode of the transistor 1760. A third line (3rd line) and one of a source electrode and a drain electrode of the transistor 1762 are electrically connected to each other, and a fourth line (4th line) and a gate electrode of the transistor 1762 are electrically connected to each other. A gate electrode of the transistor 1760 and one of the source electrode and the drain electrode of the transistor 1762 are electrically connected to the other electrode of a capacitor 1764, and a fifth line (5th line) and one of electrodes of the capacitor 1764 are electrically connected to each other.

The semiconductor device in FIG. 15C utilizes a characteristic in which the potential of the gate electrode of the transistor 1760 can be held, and thus enables data writing, holding, and reading as follows.

Writing and holding of data are described. First, the potential of the fourth line is set to a potential at which the transistor 1762 is turned on, so that the transistor 1762 is turned on. Accordingly, the potential of the third line is supplied to the gate electrode of the transistor 1760 and the capacitor 1764. That is, predetermined charge is given to the gate electrode of the transistor 1760 (writing). Here, charge for supply of a potential level or charge for supply of a different potential level (hereinafter referred to as low level charge and high level charge) is given. After that, the potential of the fourth line is set to a potential at which the transistor 1762 is turned off, so that the transistor 1762 is turned off. Thus, the charge given to the gate electrode of the transistor 1760 is held (holding).

Since the off-state current of the transistor 1762 is extremely low, the charge of the gate electrode of the transistor 1760 is held for a long time.

Next, reading of data is described. By supplying an appropriate potential (reading potential) to the fifth line while a predetermined potential (constant potential) is supplied to the first line, the potential of the second line varies depending on the amount of charge held in the gate electrode of the transistor 1760. This is because in general, when the transistor 1760 is an n-channel transistor, an apparent threshold voltage V_(th) _(—) _(H) in the case where a high level charge is given to the gate electrode of the transistor 1760 is lower than an apparent threshold voltage V_(th) _(—) _(L) in the case where a low level charge is given to the gate electrode of the transistor 1760. Here, an apparent threshold voltage refers to the potential of the fifth line, which is needed to turn on the transistor 1760. Thus, the potential of the fifth line is set to a potential V₀ which is between V_(th) _(—) _(H) and V_(th) _(—) _(L), whereby charge given to the gate electrode of the transistor 1760 can be determined For example, in the case where a high level charge is given in writing, when the potential of the fifth wiring is set to V₀ (>V_(th) _(—) _(H)), the transistor 1760 is turned on. In the case where a low level charge is given in writing, even when the potential of the fifth wiring is set to V₀ (<V_(th) _(—) _(L)), the transistor 1760 remains in an off state. Therefore, the stored data can be read by the potential of the second line.

Note that in the case where memory cells are arrayed to be used, only data of desired memory cells needs to be read. In the case of a memory cell in which reading is not performed, a potential at which the transistor 1760 is turned off, that is, a potential smaller than V_(th) _(—) _(H) may be given to the fifth wiring regardless of the state of the gate electrode of the transistor 1760. Alternatively, a potential which allows the transistor 1760 to be turned on regardless of a state of the gate electrode, that is, a potential higher than V_(th) _(—) _(L) may be applied to the fifth lines.

When a transistor having a channel formation region formed using an oxide semiconductor and having extremely small off-state current is applied to the semiconductor device in this embodiment, the semiconductor device can store data for an extremely long period. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be held for a long period even when power is not supplied (note that a potential is preferably fixed).

Further, in the semiconductor device described in this embodiment, high voltage is not needed for writing data and there is no problem of deterioration of elements. For example, unlike a conventional non-volatile memory, it is not necessary to inject and extract electrons into and from a floating gate; thus, the problem of deterioration of a gate insulating film does not occur. In other words, the semiconductor device according to an embodiment of the disclosed invention does not have a limit on the number of times of writing which is a problem in a conventional nonvolatile memory, and reliability thereof is drastically improved. Furthermore, data is written depending on the on state and the off state of the transistor, whereby high-speed operation can be easily realized.

As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided.

The structures, methods, and the like which are described in this embodiment can be combined as appropriate with any of the structures, methods, and the like which are described in the other embodiments.

Embodiment 7

In this embodiment, a semiconductor device which includes the transistor described in any of Embodiments 1 to 5, which can hold stored data even when not powered, and which does not have a limitation on the number of write cycles, and which has a structure different from the structure described in Embodiment 6 is described with reference to FIGS. 16A and 16B and FIGS. 17A and 17B.

FIG. 16A illustrates an example of a circuit configuration of a semiconductor device, and FIG. 16B is a conceptual diagram illustrating an example of a semiconductor device. First, the semiconductor device illustrated in FIG. 16A is described, and then, the semiconductor device illustrated in FIG. 16B is described below.

In the semiconductor device illustrated in FIG. 16A, a bit line BL is electrically connected to the source electrode or the drain electrode of the transistor 1762, a word line WL is electrically connected to the gate electrode of the transistor 1762, and the source electrode or the drain electrode of the transistor 1762 is electrically connected to a first terminal of a capacitor 1764.

Next, writing and holding of data in the semiconductor device (a memory cell 1850) illustrated in FIG. 16A are described.

First, the potential of the word line WL is set to a potential at which the transistor 1762 is turned on, and the transistor 1762 is turned on. Accordingly, the potential of the bit line BL is supplied to the first terminal of the capacitor 1764 (writing). After that, the potential of the word line WL is set to a potential at which the transistor 1762 is turned off, so that the transistor 1762 is turned off. Thus, the potential at the first terminal of the capacitor 1764 is held (holding).

The transistor 1762 including an oxide semiconductor has extremely low off-state current. For that reason, a potential of the first terminal of the capacitor 1764 (or a charge accumulated in the capacitor 1764) can be held for an extremely long period by turning off the transistor 1762.

Secondly, reading of data is described. When the transistor 1762 is turned on, the bit line BL which is in a floating state and the capacitor 1764 are electrically connected to each other, and the charge is redistributed between the bit line BL and the capacitor 1764. As a result, the potential of the bit line BL is changed. The amount of change in potential of the bit line BL varies depending on the potential of the first terminal of the capacitor 1764 (or the charge accumulated in the capacitor 1764).

For example, the potential of the bit line BL after charge redistribution is (C_(B)×V_(B0)+C×V)/(C_(B)+C), where V is the potential of the first terminal of the capacitor 1764, C is the capacitance of the capacitor 1764, C_(B) is the capacitance of the bit line BL (hereinafter also referred to as bit line capacitance), and V_(B0) is the potential of the bit line BL before the charge redistribution. Therefore, it can be found that assuming that the memory cell 1850 is in either of two states in which the potentials of the first terminal of the capacitor 1764 are V₁ and V₀ (V₁>V₀), the potential of the bit line BL in the case of holding the potential V₁ (=(C_(B)×V_(B0)+C×V₁)/(C_(B)+C)) is higher than the potential of the bit line BL in the case of holding the potential V₀(=(C_(B)×V_(B0)+C×V₀)/(C_(B)+C)).

Then, by comparing the potential of the bit line BL with a predetermined potential, data can be read.

As described above, the semiconductor device illustrated in FIG. 16A can hold charge that is accumulated in the capacitor 1764 for a long time because the off-state current of the transistor 1762 is extremely small. In other words, power consumption can be adequately reduced because refresh operation becomes unnecessary or the frequency of refresh operation can be extremely low. Moreover, stored data can be stored for a long time even when power is not supplied.

Next, the semiconductor device illustrated in FIG. 16B is described.

The semiconductor device illustrated in FIG. 16B includes memory cell arrays 1851 a and 1851 b including a plurality of memory cells 1850 illustrated in FIG. 16A as memory circuits in the upper portion, and a peripheral circuit 1853 in the lower portion which is necessary for operating a memory cell array 1851 (the memory cell arrays 1851 a and 1851 b). Note that the peripheral circuit 1853 is electrically connected to the memory cell array 1851.

In the structure illustrated in FIG. 16B, the peripheral circuit 1853 can be provided under the memory cell array 1851 (the memory cell arrays 1851 a and 1851 b). Thus, the size of the semiconductor device can be decreased.

It is preferable that a semiconductor material of the transistor provided in the peripheral circuit 1853 be different from that of the transistor 1762 in Embodiment 6. For example, silicon, germanium, silicon germanium, silicon carbide, gallium arsenide, or the like can be used, and a single crystal semiconductor is preferably used. Alternatively, an organic semiconductor material or the like may be used. A transistor including such a semiconductor material can operate at sufficiently high speed. Therefore, a variety of circuits (e.g., a logic circuit or a driver circuit) which needs to operate at high speed can be favorably realized by the transistor.

Note that FIG. 16B illustrates, as an example, the semiconductor device in which two memory cell arrays 1851 (the memory cell array 1851 a and the memory cell array 1851 b) are stacked; however, the number of memory cell arrays to be stacked is not limited thereto. Three or more memory cell arrays may be stacked.

Next, a specific structure of the memory cell 1850 illustrated in FIG. 16A is described with reference to FIGS. 17A and 17B.

FIGS. 17A and 17B illustrate an example of a structure of the memory cell 1850. FIG. 17A is a cross-sectional view of the memory cell 1850, and FIG. 17B is a plan view of the memory cell 1850. Here, FIG. 17A illustrates a cross section taken along line O-P and line Q-R in FIG. 17B.

The transistor 1762 in FIGS. 17A and 17B can have the same structure as the transistor in any of Embodiments 1 to 4.

An insulating film 1750 having a single-layer structure or a stacked-layer structure is provided over the transistor 1762. In addition, a conductive layer 1753 is provided in a region overlapping with the electrode film 1742 a of the transistor 1762 with the insulating film 1750 interposed therebetween, and the electrode film 1742 a, the interlayer insulating film 1735, the insulating film 1750, and the conductive layer 1753 form a capacitor 1764. That is, the electrode film 1742 a of the transistor 1762 functions as one electrode of the capacitor 1764, and the conductive layer 1753 functions as the other electrode of the capacitor 1764.

An insulating film 1752 is provided over the transistor 1762 and the capacitor 1764. Further, the memory cell 1850 and a wiring 1756 for connecting the adjacent memory cells 1850 are provided over the insulating film 1752. Although not illustrated, the wiring 1756 is electrically connected to the electrode film 1742 b of the transistor 1762 through an opening provided in the insulating film 1750, the insulating film 1752, the interlayer insulating film 1735, and the like. The wiring 1756 may be electrically connected to the electrode film 1742 b through another conductive layer provided in the opening. Note that the wiring 1756 corresponds to the bit line BL in the circuit diagram of FIG. 16A.

In FIGS. 17A and 17B, the electrode film 1742 b of the transistor 1762 can also function as a source electrode of a transistor included in an adjacent memory cell. With such a planar layout, the area occupied by the semiconductor device can be reduced; thus, higher integration can be achieved.

When the planar layout illustrated in FIG. 17A is employed, the area occupied by the semiconductor device can be reduced; thus, the degree of integration can be increased.

As described above, the plurality of memory cells is formed in the upper portion with the transistors including an oxide semiconductor. Since the off-state current of the transistor including an oxide semiconductor is small, stored data can be held for a long time owing to such a transistor. In other words, the frequency of the refresh operation can be extremely lowered, which leads to a sufficient reduction in power consumption.

A semiconductor device having a novel feature can be obtained by being provided with both a peripheral circuit including the transistor including a material other than an oxide semiconductor (in other words, a transistor capable of operating at sufficiently high speed) and a memory circuit including the transistor including an oxide semiconductor (in a broader sense, a transistor whose off-state current is sufficiently small). In addition, with a structure where the peripheral circuit and the memory circuit are stacked, the degree of integration of the semiconductor device can be increased.

As described above, a miniaturized and highly-integrated semiconductor device having high electric characteristics and a method for manufacturing the semiconductor device can be provided.

This embodiment can be implemented in appropriate combination with the structures described in the other embodiments.

Embodiment 8

In this embodiment, examples of application of the semiconductor device described in any of the above embodiments to portable devices such as cellular phones, smartphones, or e-book readers are described with reference to FIGS. 18A and 18B, FIG. 19, FIG. 20, and FIG. 21.

In a portable device such as a cellular phone, a smartphone, or an e-book reader, an SRAM or a DRAM is used to store image data temporarily. The reason why an SRAM or a DRAM is used is that a flash memory is slow in responding and is not suitable for image processing. On the other hand, an SRAM or a DRAM has the following characteristics when used for temporary storage of image data.

In an ordinary SRAM, as illustrated in FIG. 18A, one memory cell includes six transistors, that is, transistors 2001 to 2006, which are driven with an X decoder 2007 and a Y decoder 2008. The transistor 2003 and the transistor 2005, and the transistor 2004 and the transistor 2006 form inverters, which enables high-speed driving. However, because one memory cell includes six transistors, a large cell area is one disadvantage. Provided that the minimum feature size of a design rule is F, the area of a memory cell in an SRAM is generally 100 F² to 150 F². Therefore, the price per bit of an SRAM is the most expensive among memory devices.

In a DRAM, as illustrated in FIG. 18B, a memory cell includes a transistor 2011 and a storage capacitor 2012, which are driven with an X decoder 2013 and a Y decoder 2014. One cell is configured with one transistor and one capacitor and has a small area. The area of a memory cell in a DRAM is generally 10 F² or less. Note that the DRAM needs to be refreshed periodically and consumes electric power even when a rewriting operation is not performed.

On the other hand, the memory cell of the semiconductor device described in any of the above embodiments has an area of approximately 10 F² and does not need to be refreshed frequently. Therefore, the area of a memory cell can be decreased, and power consumption can be reduced.

Next, FIG. 19 is a block diagram of a portable device. The portable device illustrated in FIG. 19 includes an RF circuit 2101, an analog baseband circuit 2102, a digital baseband circuit 2103, a battery 2104, a power supply circuit 2105, an application processor 2106, a flash memory 2110, a display controller 2111, a memory circuit 2112, a display 2113, a touch sensor 2119, an audio circuit 2117, a keyboard 2118, and the like. The display 2113 includes a display portion 2114, a source driver 2115, and a gate driver 2116. The application processor 2106 includes a CPU 2107, a DSP 2108, and an interface (IF) 2109. In general, the memory circuit 2112 includes an SRAM or a DRAM. By employing the semiconductor device described in any of the above embodiments for that portion, data can be written and read at high speed and can be held for a long time, and power consumption can be sufficiently reduced.

FIG. 20 illustrates an example of using the semiconductor device described in any of the above embodiments in a memory circuit 2250 for a display. The memory circuit 2250 illustrated in FIG. 20 includes a memory 2252, a memory 2253, a switch 2254, a switch 2255, and a memory controller 2251. The memory circuit is connected to a display controller 2256 that reads and controls image data input through a signal line (input image data) and data stored in the memory 2252 and the memory 2253 (stored image data), and is also connected to a display 2257 that displays an image based on a signal input from the display controller 2256.

First, image data (input image data A) is produced by an application processor (not illustrated). The input image data A is stored in the memory 2252 through the switch 2254. Then, the image data stored in the memory 2252 (stored image data A) is transmitted to the display 2257 through the switch 2255 and the display controller 2256, and is displayed on the display 2257.

When the input image data A remains unchanged, the stored image data A is read from the memory 2252 through the switch 2255 by the display controller 2256 normally at a frequency of approximately 30 Hz to 60 Hz.

Next, for example, when a user performs an operation to rewrite a screen (i.e., when the input image data A is changed), the application processor produces new image data (input image data B). The input image data B is stored in the memory 2253 through the switch 2254. Also during that time, the stored image data A is regularly read from the memory 2252 through the switch 2255. After the completion of storing the new image data (the stored image data B) in the memory 2253, from the next frame for the display 2257, the stored image data B starts to be read, transmitted to the display 2257 through the switch 2255 and the display controller 2256, and is displayed on the display 2257. This reading operation continues until the next new image data is stored in the memory 2252.

By alternately writing and reading image data to and from the memory 2252 and the memory 2253 as described above, images are displayed on the display 2257. Note that the memory 2252 and the memory 2253 are not limited to separate memories, and a single memory may be divided and used. By employing the semiconductor device described in any of the above embodiments for the memory 2252 and the memory 2253, data can be written and read at high speed and held for a long time, and power consumption can be sufficiently reduced.

FIG. 21 is a block diagram of an e-book reader. FIG. 21 includes a battery 2301, a power supply circuit 2302, a microprocessor 2303, a flash memory 2304, an audio circuit 2305, a keyboard 2306, a memory circuit 2307, a touch panel 2308, a display 2309, and a display controller 2310.

Here, the semiconductor device described in any of the above embodiments can be used for the memory circuit 2307 in FIG. 21. The memory circuit 2307 has a function of temporarily storing the contents of a book. For example, a user may use a highlight function. In some cases, a user wants to mark a specific portion while reading e-book. This marking function is called a highlight function and is used to make a difference from the other portions by changing the display color, underlining, making characters bold, changing the font of characters, or the like. The function makes it possible to store and hold data of a portion specified by a user. In order to store the data for a long time, the data may be copied to the flash memory 2304. Also in such a case, the semiconductor device described in any of the above embodiments is used, whereby writing and reading of data can be performed at high speed, data can be stored for a long time, and power consumption can be sufficiently reduced.

As described above, the portable devices described in this embodiment each incorporate the semiconductor device according to any of the above embodiments. Therefore, it is possible to obtain a portable device which is capable of reading data at high speed, holding data for a long time, and reducing power consumption.

The structures, methods, and the like which are described in this embodiment can be combined as appropriate with any of the structures, methods, and the like which are described in the other embodiments.

Embodiment 9

A semiconductor device disclosed in this specification and the like can be applied to a variety of electronic devices (including game machines). Examples of electronic devices are a television set (also referred to as a television or a television receiver), a monitor of a computer or the like, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game machine, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like. Examples of electronic devices each including the liquid crystal display device described in the above embodiment are described.

FIG. 22A illustrates a laptop personal computer, which includes a main body 2501, a housing 2502, a display portion 2503, a keyboard 2504, and the like. By applying the semiconductor device described in any of the above embodiments, the laptop personal computer can have high reliability.

FIG. 22B illustrates a personal digital assistant (PDA) including a display portion 2513, an external interface 2515, an operation button 2514, and the like in a main body 2511. A stylus 2512 is included as an accessory for operation. By applying the semiconductor device described in any of the above embodiments, the portable digital assistant (PDA) can have higher reliability.

FIG. 22C illustrates an example of an e-book reader. For example, an e-book reader 2520 includes two housings, a housing 2521 and a housing 2523. The housing 2521 and the housing 2523 are combined with a hinge 2522 so that the e-book reader 2520 can be opened and closed with the hinge 2522 as an axis. With such a structure, the e-book reader 2520 can operate like a paper book.

A display portion 2525 and a display portion 2527 are incorporated in the housing 2521 and the housing 2523, respectively. The display portion 2525 and the display portion 2527 may display one image or different images. In the case where the display portion 2525 and the display portion 2527 display different images, for example, a display portion on the right side (the display portion 2525 in FIG. 22C) can display text and a display portion on the left side (the display portion 2527 in FIG. 22C) can display images. By applying the semiconductor device described in any of the above embodiments, the e-book reader 2520 can have high reliability.

In FIG. 22C, the housing 2521 includes an operation portion and the like as an example. For example, the housing 2521 is provided with a power switch 2526, operation keys 2528, a speaker 2529, and the like. With the operation key 2528, pages can be turned. Note that a keyboard, a pointing device, or the like may also be provided on the surface of the housing, on which the display portion is provided. Furthermore, an external connection terminal (an earphone terminal, a USB terminal, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Moreover, the e-book reader 2520 may have a function of an electronic dictionary.

The e-book reader 2520 may have a configuration capable of wirelessly transmitting and receiving data. Through wireless communication, desired book data or the like can be purchased and downloaded from an electronic book server.

FIG. 22D illustrates a smartphone, which includes a housing 2530, a button 2531, a microphone 2532, a display portion 2533 provided with a touch panel, a speaker 2534, and a camera lens 2535 and functions as a mobile phone. By applying the semiconductor device described in any of the above embodiments, the smartphone can have high reliability.

The display direction of the display portion 2533 can be changed depending on a usage pattern. Since the camera lens 2535 is provided on the same plane as the display portion 2533, videophone is possible. The speaker 2534 and the microphone 2532 can be used for operations such as video calls, sound recording, and playback without being limited to the voice call function.

An external connection terminal 2536 can be connected to an AC adapter and various types of cables such as a USB cable, and charging and data communication with a personal computer are possible. Furthermore, a large amount of data can be stored and moved by inserting a storage medium into the external memory slot (not illustrated).

Further, in addition to the above functions, an infrared communication function, a television reception function, or the like may be provided.

FIG. 22E illustrates a digital video camera which includes a main body 2541, a display portion 2542, an operation switch 2543, a battery 2544, and the like. By applying the semiconductor device described in any of the above embodiments, the digital video camera can have high reliability.

FIG. 22F illustrates an example of a television set. In a television set 2550, a display portion 2553 is incorporated in a housing 2551. The display portion 2553 can display images. Here, the housing 2551 is supported by a stand 2555. By applying the semiconductor device described in any of the above embodiments, the television set 2550 can have high reliability.

The television set 2550 can be operated by an operation switch of the housing 2551 or a separate remote controller. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller.

Note that the television set 2550 is provided with a receiver, a modem, and the like. With use of the receiver, general television broadcasting can be received. Moreover, when the display device is connected to a communication network with or without wires via the modem, one-way (from a sender to a receiver) or two-way (between a sender and a receiver or between receivers) information communication can be performed.

The structures, methods, and the like which are described in this embodiment can be combined as appropriate with any of the structures, methods, and the like which are described in the other embodiments.

EXAMPLE 1

In this example, an oxide semiconductor film containing silicon was formed, and the result of measuring the sheet resistance of the oxide semiconductor film and the result of composition analysis of the oxide semiconductor film by using X-ray photoelectron spectroscopy (XPS) are described.

In this example, samples were manufactured by depositing oxide semiconductor films over glass substrates by a sputtering method at different gas flow proportion (a gas containing oxygen at 33% and a gas containing oxygen at 100%) with the use of targets to which SiO₂ was added at different concentrations (0 wt. %, 2 wt. %, and 5 wt. %).

As sputtering targets, an IGZO target (In:Ga:Zn=1:1:1 [atomic ratio]), an IGZO target (In:Ga:Zn=1:1:1 [atomic ratio]) to which SiO₂ was added at 2 wt. %, and an IGZO target (In:Ga:Zn=1:1:1 [atomic ratio]) to which SiO₂ was added at 5 wt. % were used.

With the targets, the oxide semiconductor films were deposited by sputtering with the flow rate where O₂ gas=10 sccm or Ar/O₂=10 sccm/5 sccm. The other conditions were the same in all of Sample and were as follows: substrate temperature, 200° C.; deposition power, 100 W (DC power source); deposition pressure, 0.4 Pa; and thickness, 100 nm.

That is, the following samples were manufactured: Sample L in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 100% with the use of the target to which SiO₂ was not added; Sample M in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 100% with the use of the target to which SiO₂ was added at 2 wt. %; Sample N in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 100% with the use of the target to which SiO₂ was added at 5 wt. %; Sample O in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 33% with the use of the target to which SiO₂ was not added; Sample P in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 33% with the use of the target to which SiO₂ was added at 2 wt. %; and Sample Q in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 33% with the use of the target to which SiO₂ was added at 5 wt. %.

Further, heat treatment was performed on Sample L to Sample Q by introducing them into an electric furnace using a resistance heater. The heat treatment was performed at 450° C. in an N₂ atmosphere for one hour and then at 450° C. in an O₂ atmosphere for one hour.

Sheet resistances of Sample L to Sample Q on which the above treatment was performed were measured. Measurement results of the sheet resistances of Sample L to Sample Q are shown in a graph of FIG. 23. The vertical axis of the graph of FIG. 23 represents a sheet resistance (Ω/square). The horizontal axis thereof represents a concentration of SiO₂ in a target (wt. %).

From the graph of FIG. 23, as the SiO₂ concentrations in the targets increase, the sheet resistances of the oxide semiconductor films tend to increase. The sheet resistances of Sample L and Sample O each of which was manufactured using a target to which SiO₂ was not added were approximately 8×10⁵ Ω/square to 1×10⁶ Ω/square, which were sheet resistance applicable values for an active layer of a transistor or the like. In addition, the sheet resistances of Sample M and Sample P each of which was manufactured using a target to which SiO₂ was added at 2 wt. % were approximately 1×10⁶ Ω/square to 3×10⁶ Ω/square, which were sheet resistance applicable values for an active layer of a transistor or the like. However, the sheet resistances of Sample N and Sample Q each of which was manufactured using a target to which SiO₂ was added at 5 wt. % were larger than the measurement limit; thus, when the oxide semiconductor films of Sample N and Sample Q are used as an active layer of a transistor of the like, the on-state current might be decreased.

According to the above, a SiO₂ concentration in a target used in a deposition of an oxide semiconductor film of a transistor is preferably small and may be about 2 wt. % or less, for example.

Further, in this example, a sample was manufactured by depositing an oxide semiconductor film over a silicon substrate in the condition similar to that in Sample M and Sample N and the composition of the sample was analyzed by XPS.

As sputtering targets, an IGZO target (In:Ga:Zn=1:1:1 [atomic ratio]) to which SiO₂ was added at 2 wt. % and an IGZO target (In:Ga:Zn=1:1:1 [atomic ratio]) to which SiO₂ was added at 5 wt. % were used.

The deposition conditions were as follows: gas flow rate, O₂=10 sccm; substrate temperature, 200° C.; deposition power, 100 W (DC power source); deposition pressure, 0.4 Pa; and thickness, 15 nm.

That is, the following samples were manufactured: Sample R in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 100% with the use of the target to which SiO₂ was added at 2 wt. %; and Sample S in which an oxide semiconductor film was deposited in an atmosphere containing oxygen at 100% with the use of the target to which SiO₂ was added at 5 wt. %;

Results of the composition analysis of Sample R and Sample S by XPS were as follows: the silicon concentrations of oxide semiconductor films of Sample R and Sample S were 1.1 at. % and 2.6 at. %, respectively. That is, the silicon concentration of the oxide semiconductor film deposited by using a target to which SiO₂ was added at 2 wt. % was 1.1 at. %, and the silicon concentration of an oxide semiconductor film deposited by using a target to which SiO₂ was added at 5 wt. % was 2.6 at. %.

As described above, when impurities such as silicon enter, by mixing or the like, a portion of an oxide semiconductor film which is in the vicinity of an interface with a gate insulating film, the resistance of a channel formation region is increased and thus on-state current of the transistor might be decreased. Accordingly, it is important to reduce the silicon concentration in the portion of the oxide semiconductor film which is in the vicinity of the interface with the gate insulating film.

EXAMPLE 2

As described in Embodiment 1, a CAAC-OS film is preferably used as the oxide semiconductor film described in any of the above embodiments. However, there is a concern that the crystal structure of the CAAC-OS film is changed by silicon mixed into the CAAC-OS film.

Thus, in this example, calculation was performed to examine the concentration of silicon mixed into the oxide semiconductor film having high crystallinity such as a CAAC-OS film, at which the crystal structure of the oxide semiconductor film is distorted. The result of the calculation will be described.

In the calculation in this example, “classical molecular dynamics method” was used as a calculation method, and “SCIGRESS ME” manufactured by FUJITSU LIMITED was used for the calculation.

Further, a model of a single crystal structure of InGaZnO₄ including 1680 atoms (see FIG. 24A) was used as the film having high crystallinity. Note that the density of the model was 6.36 g/cm³.

With the use of the model, at a constant temperature and under a constant pressure (pressure: 1 atm, temperature: 300° C.), a sample in which two In atoms, two Ga atoms, two Zn atoms, and eight oxygen atoms were replaced with Si atoms (hereinafter this sample is referred to as Sample A) was obtained, and a sample in which three In atoms, three Ga atoms, three Zn atoms, and twelve oxygen atoms were replaced with Si atoms (hereinafter this sample is referred to as Sample B) was obtained. The initial structure and a structure after 2 nsec of Sample A and Sample B were calculated.

Note that 0.83 at. % (0.52 wt. %) of the entire structure (1680 atoms) were replaced with Si atoms in Sample A, while 1.25 at. % (0.79 wt. %) of the entire structure were replaced with Si atoms in Sample B.

First, FIGS. 24A and 24B show the structure of Sample A and the structure of Sample B in an initial state (0 nsec), and FIG. 24C shows the structure of a sample which is not subjected to replacement of In atoms, Ga atoms, Zn atoms, and O atoms with Si atoms (hereinafter this sample is referred to as Sample C).

FIGS. 24A to 24C show that, in the initial state, both Sample A and Sample B have high crystallinity as in Sample C.

Next, the crystal states of Sample A and Sample B after 2 nsec are described with reference to FIGS. 25A and 25B.

FIG. 25A shows the crystal state of Sample A after 2 nsec. The radial distribution function g(r) of the structure was calculated for the purpose of examining whether or not the structure had crystallinity.

Note that the above-described “radial distribution function g(r)” is a function representing the probability density of atoms existing at a distance of r from one atom. As the correlation between atoms disappears, g(r) becomes closer to 1.

FIG. 25B shows the calculation result of a radial distribution function of Sample A. In FIG. 25B, the horizontal axis represents a distance r (nm), and the vertical axis represents a radial distribution function g(r). Note that in FIG. 25B, a solid line represents a radial distribution function of Sample A, and a dashed line represents a radial distribution function of Sample C.

FIG. 25B shows that, in the radial distribution function of Sample A after 2 nsec, there is an order (in other words, there is a peak) even when a distance r (nm) is increased, as in the radial distribution function of Sample C. This shows that the crystallinity of Sample A is kept.

FIG. 26A shows the crystal state of Sample B after 2 nsec, and FIG. 26B shows the calculation result of a radial distribution function g(r) of the structure. Note that in FIG. 26B, a solid line represents a radial distribution function of Sample B, and a dashed line represents a radial distribution function of Sample C.

FIG. 26A shows that the structure of Sample B after 2 nsec is obviously changed as compared to the structure of Sample B in the initial state which is shown in FIG. 24B.

As also seen in FIG. 26B showing the radial distribution function of Sample B after 2 nsec, when a distance r (nm) is increased, the order is lost and thus a flat line is given (in other words, a peak disappears). This shows that the crystallinity of Sample B is not kept (i.e., Sample B is made amorphous).

In this specification, the following description is made: a semiconductor device including an oxide semiconductor film as a semiconductor layer preferably includes a region in which a concentration of silicon distributed from an interface with a gate insulating film toward the inside of the oxide semiconductor film is lower than or equal to 1.1 at. %. From the results of this example, it is confirmed that, in the case where an oxide semiconductor film having high crystallinity such as a CAAC-OS film is used as a semiconductor layer, a region in which a concentration of silicon distributed from the interface with the gate insulating film toward the inside of the oxide semiconductor film is lower than or equal to 0.83 at. % is more preferably included.

This application is based on Japanese Patent Application serial no. 2011-227198 filed with Japan Patent Office on Oct. 14, 2011, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: an oxide semiconductor film; a gate insulating film over the oxide semiconductor film, the gate insulating film comprising an oxide comprising silicon; and a gate electrode over the gate insulating film, the gate electrode overlapping the oxide semiconductor film, wherein the oxide semiconductor film comprises a first region in which a concentration of silicon distributed from an interface with the gate insulating film toward an inside of the oxide semiconductor film is lower than or equal to 1.1 at. %.
 2. The semiconductor device according to claim 1, wherein the first region is in contact with the gate insulating film and have a thickness less than or equal to 5 nm.
 3. The semiconductor device according to claim 1, wherein the oxide semiconductor film further comprises a second region under the first region, wherein a concentration of silicon contained in the second region is lower than the concentration of silicon contained in the first region.
 4. The semiconductor device according to claim 1, wherein the concentration of silicon contained in the first region is lower than or equal to 0.83 at. %.
 5. The semiconductor device according to claim 1, wherein the concentration of silicon contained in the first region is lower than or equal to 0.1 at. %.
 6. The semiconductor device according to claim 1, wherein the gate insulating film contains carbon, and wherein a concentration of carbon in the first region is lower than or equal to 1.0×10²⁰ atoms/cm³.
 7. The semiconductor device according to claim 1, wherein the oxide semiconductor film has crystallinity.
 8. The semiconductor device according to claim 1, wherein the oxide semiconductor film has an amorphous structure.
 9. A method of manufacturing a semiconductor device, the method comprising the steps of: forming an oxide semiconductor film; forming a gate insulating film comprising an oxide comprising silicon, over the oxide semiconductor film; and forming a gate electrode over the gate insulating film, wherein a first layer of the gate insulating film is formed so that a concentration of silicon distributed from an interface with the gate insulating film toward an inside of the oxide semiconductor film is lower than or equal to 1.1 at. %, and wherein after formation of the first layer of the gate insulating film, a second layer of the gate insulating film is formed under a condition that a deposition rate of the second layer of the gate insulating film is higher than a deposition rate of the first layer of the gate insulating film.
 10. The method according to claim 9, wherein the concentration of silicon contained in the oxide semiconductor film is lower than or equal to 0.83 at. %.
 11. The method according to claim 9, wherein the concentration of silicon contained in the oxide semiconductor film is lower than or equal to 0.1 at. %.
 12. The method according to claim 9, wherein the gate insulating film contains carbon, and wherein a concentration of carbon in the oxide semiconductor film is lower than or equal to 1.0×10²⁰ atoms/cm³.
 13. The method according to claim 9, wherein the oxide semiconductor film has crystallinity.
 14. The method according to claim 9, wherein the oxide semiconductor film has an amorphous structure. 